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Longitudinal Study of Dust and Airborne Endotoxin in the Home.



To characterize the seasonal variability of endotoxin Endotoxin

A biologically active substance produced by bacteria and consisting of lipopolysaccharide, a complex macromolecule containing a polysaccharide covalently linked to a unique lipid structure, termed lipid A.
 levels, we measured endotoxin in dust from the bed, bedroom floor, and kitchen floor in 20 homes, and in air from the bedroom in 15 of the homes. All homes were located in the greater Boston Greater Boston is the area of the Commonwealth of Massachusetts surrounding the city of Boston, Massachusetts. While Metro Boston tends to be the "Inner Core" surrounding the City of Boston, Greater Boston overlaps the North and South Shores, as well as the MetroWest region. , Massachusetts, area and were sampled each month from April 1995 to June 1996. Outdoor air was collected at two locations. We found greater within-home than between-home variance for bedroom floor, kitchen floor, and airborne endotoxin. However, the reverse was true for bed dust endotoxin. Thus, studies using single measurements of dust endotoxin are most likely to reliably distinguish between homes if bed dust is sampled. Dust endotoxin levels were not significantly associated with airborne endotoxin. Airborne endotoxin was significantly (p = 0.04) and positively associated with absolute humidity absolute humidity
n.
The weight of water vapor present per unit volume of a gas or a mixture of gases.
 in a mixed-effect model adjusting for a random home effect and fixed effect of sampling month and home characteristics. This finding implies that indoor humidity may be an important factor controlling endotoxin exposure. We found a significant (p [is less than] 0.05) seasonal effect in kitchen floor dust (spring [is greater than] fall) and bedroom airborne endotoxin (spring [is greater than] winter), but not in the other indoor samples. We found significant seasonal pattern in outdoor airborne endotoxin (summer [is greater than] winter). Key words: endotoxin, house dust, humidity, indoor air pollution, seasonal variability, temperature. Environ Health Perspect 108:1023-1028 (2000). [Online 5 October 2000]

http://ehpnet1.niehs.nih.gov/docs/2000/108p1023-1028park/abstract.html

Endotoxin, a proinflammatory component of the outer membrane The outer membrane refers to the outside membranes of Gram-negative bacteria, the chloroplast, or the mitochondria. It is used to maintain the shape of the organelle contained within its structure, and it acts as a barrier against certain dangers.  of gram-negative bacteria that produces airway airway /air·way/ (-wa)
1. the passage by which air enters and leaves the lungs.

2. a device for securing unobstructed respiration.
 inflammation when inhaled in·hale  
v. in·haled, in·hal·ing, in·hales

v.tr.
1. To draw (air or smoke, for example) into the lungs by breathing; inspire.

2.
, is present in house dust (1-5), and thus exposure to endotoxin is common to everyone. Exposure to endotoxin, however, is not well characterized; for example, little is known about seasonal patterns, between- and within-home variability of endotoxin levels, correlation with home climatic factors, and sources of endotoxin.

Research on seasonal patterns of indoor and outdoor airborne endotoxin levels may be useful in understanding the seasonal pattern of respiratory disease Noun 1. respiratory disease - a disease affecting the respiratory system
respiratory disorder, respiratory illness

adult respiratory distress syndrome, ARDS, wet lung, white lung - acute lung injury characterized by coughing and rales; inflammation of the
 (6-8) and in establishing a strategy for measurement of home endotoxin. Rizzo et al. (3) reported lower levels of house dust endotoxin during the winter on the basis of the repeated measurements in a group of homes over 1 year. In an occupational setting, DeLucca and Palmgren (9) reported seasonal variation of airborne endotoxin in respirable respirable /res·pir·a·ble/ (re-spir´ah-b'l)
1. suitable for respiration.

2. small enough to be inhaled.


res·pi·ra·ble
adj.
1. Fit for breathing, as air.
 dust and in settled grain dusts sampled over 16 months at two grain terminals on the lower Mississippi River

Main article: Mississippi River
The Lower Mississippi River is the portion of the Mississippi River downstream of Cairo, Illinois. From the confluence of the Ohio River and Upper Mississippi River at Cairo, the Lower flows just under 1600
. To our knowledge however, there are no published reports about seasonal variation of airborne endotoxin in homes or in the outdoor, ambient air.

In occupational epidemiologic studies epidemiologic study A study that compares 2 groups of people who are alike except for one factor, such as exposure to a chemical or the presence of a health effect; the investigators try to determine if any factor is associated with the health effect , information about within- and between-worker variability has been used (10-14) to establish optimal strategies for assessing exposure to workplace contaminants. Likewise, we may also be able to use information about variance components (between- and within-home variance) in endotoxin level to improve assessment of exposure to endotoxin at home. However, there are no previous reports of the within- and between-home variance components of endotoxin.

In this study we measured dust endotoxin in 20 homes and airborne endotoxin in 15 Of those homes at monthly intervals for up to 13 months. With these repeated measurements, we analyzed within- and between-home variance components. We investigated the presence of seasonal and indoor climatic influences on endotoxin using the repeated within-home measurements. We also examined seasonal patterns of outdoor airborne endotoxin levels.

Methods

Study cohort. This study is a component of a longitudinal exposure measurement study designed to characterize seasonal variation in home allergen allergen /al·ler·gen/ (al´er-jen) an antigenic substance capable of producing immediate hypersensitivity (allergy).allergen´ic

pollen allergen
, fungus (15), and endotoxin levels. We recruited 20 subjects from the faculty, staff, and students at the Harvard School of Public Health The Harvard School of Public Health is (colloquially, HSPH) is one of the professional graduate schools of Harvard University. Located in Longwood Area of the Boston, Massachusetts neighborhood of Mission Hill, next to Harvard Medical School and Cambridge, Massachusetts,  who lived in the greater Boston, Massachusetts, area, who did not plan to move during the study period, and who agreed to help by collecting samples and measuring other environmental factors in their homes.

Environmental measurements. Each participant in the 20 homes collected three dust samples (bedroom bed, bedroom floor, and kitchen floor) on prescheduled days every month from April 1995 through July 1996. Participants in 15 homes also collected air samples, (4 ft above the floor and 1 ft from the nearest wall in the bedroom for 24 hr) every month before dust samples were taken. Vacuuming for collecting dust from the floors and beds followed the published protocol (15). Briefly, participants collected settled dust from all layers of bedding by vacuuming for 5 min with a modified Eureka Mighty-Mite II canister vacuum cleaner vacuum cleaner, mechanical device using a draft of air to remove dust, loose dirt, or other particulate matter from dry surfaces. It is especially useful on highly textured surfaces, such as carpets and upholstery, that are difficult to clean by wiping or brushing.  (Model 3621; The Eureka Co., Bloomington, IN) fitted with a 19 x 90 mm cellulose cellulose, chief constituent of the cell walls of plants. Chemically, it is a carbohydrate that is a high molecular weight polysaccharide. Raw cotton is composed of 91% pure cellulose; other important natural sources are flax, hemp, jute, straw, and wood.  extraction thimble thimble,
n See coping.

thimble, ionization chamber,
n See chamber, ionization, thimble.
 (Whatman International, Ltd., Maidstone, England). Using a separate thimble, a measured area of the bedroom floor (either 1 or 2 [m.sup.2]) was vacuumed for 5 min; a third thimble was used to collect kitchen samples when the floor around all edges of cabinets, inside the cabinet under the sink, and around and behind the refrigerator was vacuumed for a total of 5 min. If there was a rug on the kitchen or bedroom floor, it was vacuumed for at least 1 min. The collected dust was weighed in the laboratory and sifted through a 425-[micro]m mesh sieve, and the fine dust was reweighed and separated into aliquots for various analyses: allergens, culturable fungi, and endotoxin. Endotoxin was only assayed if there was sufficient fine dust after dust was used for all other assays.

Indoor air samples were collected from the bedroom of 15 homes. Air was sampled for 24 hr using a Gilian pump (model HFS (Hierarchical File System) The file system used in the Macintosh. The first version, known as "Mac OS Standard," was introduced in 1985. HFS+, an enhanced version, came out in 1998 in preparation for the upcoming Mac OS X operating system.  513A; Gilian Instrument Corp., West Caldwell West Caldwell, borough (1990 pop. 10,422), Essex co., NE N.J., a residential suburb of Newark and New York City; inc. 1904. It has some light manufacturing. , NJ) attached to a filter cassette assembled with a 0.4-[micro]m preweighted polycarbonate A category of plastic materials used to make a myriad of products, including CDs and CD-ROMs.  filter. Outdoor air samples were collected weekly from two locations, one urban and one suburban, during spring, summer, and fall, and at least every other week during the winter. Air was sampled for 2-3 days on each occasion. The urban site was located outside a first-floor apartment beside the Charles River Charles River

River, eastern Massachusetts, U.S. The longest river wholly in the state, it flows into Boston Bay after a course of about 80 mi (130 km). Navigable for about 7 mi (11 km), its estuary separates the cities of Boston and Cambridge.
 in Cambridge, Massachusetts This article is about the city of Cambridge in Massachusetts. For the English university town, see Cambridge, England. For other places, see Cambridge (disambiguation).
Cambridge, Massachusetts is a city in the Greater Boston area of Massachusetts, United States.
, 6 ft above the ground and 2 inches from the building. The suburban site was located outside of a single-family house, 6 ft above ground and 8 inches from the building, adjacent to a pasture and downwind down·wind  
adv.
In the direction in which the wind blows.



downwind
 of a commercial vegetable farm using large quantities of manure manure, term used in the United States to refer to excreta of animals, with or without added bedding; also called barnyard manure. In other countries the term often refers to any material used to fertilize the soil.  in Lexington, Massachusetts Lexington is a town in Middlesex County, Massachusetts, United States. The population was 30,355 at the 2000 census.

The town is famous for being the site of the opening shots of the Battles of Lexington and Concord, the first engagement of the American Revolution.
. Each air-sampling assembly was precalibrated at 2 L/min flow rate with a Gilian soap bubble soap bubble An adjective referring to a dilated, smooth-contoured cyst-like or ballooned, occasionally loculated space(s). See Physaliferous Bone radiology An expansile, often eccentric, vaguely trabeculated space with a thin, sclerotic, sharply defined margin,  flowmeter See flow meter.  (P/N (Part/Number) Common shorthand for part number.  800286; Gilian Instrument Corp.) before sampling, and was postcalibrated with the same instrument after sampling. After sampling, filters were weighed for total suspended particulate par·tic·u·late
adj.
Of or occurring in the form of fine particles.

n.
A particulate substance.



particulate

composed of separate particles.
 (TSP TSP - travelling salesman problem ) analysis with an electrobalance (model Cahn 21; Cahn Instrument Inc., Cerritos, CA) at 65-75 [degrees] F, 35-45% relative humidity relative humidity
n.
The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage.
, then assayed for endotoxin.

Participants completed a questionnaire about home characteristics each month. They also measured wet-bulb and dry-bulb temperature The dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. In construction, it is an important consideration when designing a building for a certain climate.  in the air 2-3 inches above the bedroom floor and on the surface of the bed and bedroom floor with a Microscanner D-501 (Exergen Coportion, Newton, MA). Relative humidity in air and on the surfaces [water activity: amount of water available for microorganisms on the sampling surface (16)] was calculated based on measured dry-bulb and wet-bulb temperature Wet-bulb temperature - there are several meanings of this term:
  1. The temperature read from a wet bulb thermometer,
  2. Isobaric wet-bulb temperature: the temperature an air parcel would have if cooled adiabatically to saturation at constant pressure by evaporation of water
, and absolute humidity (grams per kilogram kilogram, abbr. kg, fundamental unit of mass in the metric system, defined as the mass of the International Prototype Kilogram, a platinum-iridium cylinder kept at Sèvres, France, near Paris. ) was estimated using a psychrometric chart.

Endotoxin assay. Dust endotoxin and airborne endotoxin were assayed as previously described (2,17). Results were reported in endotoxin units with reference to the EC5 or EC6 reference standard endotoxin [U.S. Pharmacopoeia pharmacopoeia or pharmocopeia (fär'məkəpē`ə), authoritative publication designating the properties, action, use, dosage, and standards of strength and purity of drugs. , Inc., Rockville, MD; 1 ng EC5 and EC6 = 10 endotoxin units (EU)]. In a previous report (2), we showed that lots of Limulus amebocyte lysate Limulus Amoebocyte Lysate (LAL) is an aqueous extract of blood cells (amoebocytes) from the horseshoe crab, Limulus polyphemus. LAL reacts with bacterial endotoxin or lipopolysaccharide (LPS), which is a membrane component of Gram negative bacteria.  (LAL LAL Laughing A Lot
LAL Los Angeles Lakers
LAL Lithuanian Airlines
LAL Lightning Activity Level (used for wildfire prediction)
LAL Limulus Amoebocyte Lysate
LAL Latitude and Longitude
LAL Live and Learn
) differed in their sensitivity to environmental endotoxin. Therefore, we tested and compared the sensitivity of the LAL lots used in this study to house dust-associated endotoxin and adjusted the final estimates so that all data were on the same scale.

Blank filters for quality control of air sampling were subjected to all the procedures of calibration and storage (on average, four blank filters per month). If the blank filters indicated possible contamination during calibration, shipping, and storage, we excluded air samples collected between 1 day before the date of the contaminated contaminated,
v 1. made radioactive by the addition of small quantities of radioactive material.
2. made contaminated by adding infective or radiographic materials.
3. an infective surface or object.
 blank and the date of next dean blank. Out of 334 air samples, 53 filters were excluded from data analysis because of contamination that occurred during calibration when soap solution in the bubble generator became contaminated with gram-negative bacteria.

Data analysis. The normality normality, in chemistry: see concentration.  of distribution for measured and log-transformed dust and airborne endotoxin was evaluated by the Shapiro-Wilk normality test In statistics, normality tests are used to determine whether a random variable is normally distributed, or not.

One application of normality tests is to the residuals from a linear regression model.
 (18). All log-transformed endotoxin measurements were approximately Gaussian except for those of log-transformed bedroom floor dust endotoxin, which were symmetrical. Therefore, all data analyses were performed with log-transformed data. For multiple comparisons, Scheffe's method (19) was used to correct p-values.

To analyze within- and between-home variance components, we applied the random effects models In statistics, a random effect(s) model, also called a variance components model is a kind of hierarchical linear model. It assumes that the data describe a hierarchy of different populations whose differences are constrained by the hierarchy.  described by Rappaport and colleagues (11,12,20,21). Between- and within-home variance components for airborne endotoxin sampled in bedroom, and endotoxin in dust from bed, bedroom floor, and kitchen floor were analyzed in mixed-effect models with a random home effect and a fixed effect of season (spring: April-May; summer: June-August; fall: September-October; winter: November-March). The mixed-effect models were further adjusted for time-varying home characteristics (operating humidifier humidifier,
n a device for adding moisture to dry air inside the home to help counteract the reduction in saliva that often occurs as a result of hyposalivation, radiation therapy, or other treatments that cause xerostomia.
, windows open, and indoor climate parameters) to examine whether within-home variance could be explained by these factors.

Based on the estimated within- and between-home variance components, the geometric standard deviation In probability theory and statistics, the geometric standard deviation describes how spread out are a set of numbers whose preferred average is the geometric mean. If the geometric mean of a set of numbers is denoted as μg  (GSD GSD German Shepherd Dog
GSD Graduate School of Design
GSD Glycogen Storage Disease
GSD General Services Division
GSD Gundam Seed Destiny (anime)
GSD Ground Sample Distance
GSD Geometric Standard Deviation
) of home endotoxin was calculated by taking the antilog an·ti·log  
n.
An antilogarithm.

Noun 1. antilog - the number of which a given number is the logarithm
antilogarithm
 of the square root of each variance. We also computed the ratio of within-home to between-home variance and the within-home correlation coefficient Correlation Coefficient

A measure that determines the degree to which two variable's movements are associated.

The correlation coefficient is calculated as:
 [the ratio of between-home to sum of within- and between-home variance, a measure of the reproducibility of repeated measurements (22)].

To examine the correlations between the average home climatic parameters and airborne and dust endotoxin (bed, bedroom floor, and kitchen floor) levels, we averaged all measured values over 13 months from each sampling location within homes. Because the number of homes in this study was relatively small (15 homes with airborne endotoxin and 20 homes with dust endotoxin), we calculated Spearman spear·man  
n.
A man, especially a soldier, armed with a spear.
 correlation coefficients.

We used regression models controlling for a random home effect and for the fixed effects of sampling month and room to examine the association of time-varying home climatic parameters with dust endotoxin levels. A regression model controlling for a random home effect and the fixed effect of sampling month was also fit to examine the association of time-varying climatic parameters with airborne endotoxin. We identified home characteristics significantly associated with endotoxin and adjusted for these factors in our final multivariate The use of multiple variables in a forecasting model.  models to ensure that the association was not confounded by these home characteristics.

We analyzed seasonal variation in the dust and airborne endotoxin measurements using mixed linear regression Linear regression

A statistical technique for fitting a straight line to a set of data points.
 models with a random home effect and a fixed season effect (19). In these mixed models, we categorized cat·e·go·rize  
tr.v. cat·e·go·rized, cat·e·go·riz·ing, cat·e·go·riz·es
To put into a category or categories; classify.



cat
 sampling month into four seasons based on the categorization of season used by Chew et al. (15) in their analysis of antigen levels in these homes. We also adjusted for time-varying home characteristics to determine whether they could explain the seasonal effect on endotoxin levels. To graphically examine time trends in relation to endotoxin levels, we applied the smoothing cubic spline In computer graphics, a smooth curve that runs through a series of given points. The term is often used to refer to any curve, because long before computers, a spline was a flat, pliable strip of wood or metal that was bent into a desired shape for drawing curves on paper. See Bezier and B-spline.  technique with 8 degrees of freedom (23).

Results

Distribution and variance components of endotoxin levels. The distributions of endotoxin in bedroom bed and kitchen floor dust, and distributions of airborne endotoxin and TSP were approximately lognormal log·nor·mal  
adj. Mathematics
Of, relating to, or being a logarithmic function with a normal distribution.



log
 (Shapiro-Wilk normality test: p = 0.23, 0.27, 0.89, and 0.15, respectively). Endotoxin levels (Table 1) in kitchen floor and bedroom floor dust were significantly higher (p [is less than] 0.05) than in dust from beds. Endotoxin levels in kitchen floor dust were also significantly higher (p [is less than] 0.05) than in bedroom floor dust.
Table 1. Distribution of endotoxin level and TSP.

                     Dust (EU/mg)            Air             TSP
Parameter         BB      BF      KF    (EU/[m.sup.3])   ([micro]g
                                                         /[m.sup.3])

GM               43.5    76.8   105.4       0.64              52.9
Median           43.7    80.2   104.7       0.64              57
GSD               2.9     1.9     2.6       2.6               1.7
Minimum           2.8     5.2     4.2       0.02               9.3
Maximum        1057.2   459.5   844.4      19.82             173.8
IQR              53.8    53.4   147.0       0.75              41.2
Number
  of samples    118     200     128       142                195

Abbreviations: BB, bedroom bed BF, bedroom floor; KF, kitchen floor; GM,
geometric mean; IQR, interquartile range.


The home-specific medians of repeated endotoxin measurements for bed and kitchen floor dust were within two orders of magnitude across all homes studied (Figures 1 and 2). The range of median bedroom floor dust endotoxin levels covered one order of magnitude A change in quantity or volume as measured by the decimal point. For example, from tens to hundreds is one order of magnitude. Tens to thousands is two orders of magnitude; tens to millions is three orders of magnitude, etc.  (Figure 3). The crude GSD for endotoxin in bed dust was larger than the GSDs for bedroom floor and kitchen floor dust (Table 1). However, when the total variance was partitioned into within-home and between-home variance, the between-home GSD was greater than within-home GSD for bed dust endotoxin; on the other hand, within-home GSDs were greater for endotoxin in floor dust from bedrooms and kitchens (Table 2). When all sources of house dust endotoxin were analyzed together (pooled data) in a mixed-effect model with a random home effect controlling for the fixed effects of season and room, the within-home GSD was greater than the between-home GSD.

[GRAPHS OMITTED]
Table 2. Variance components of log-transformed home endotoxin level.

                                                        GSD

Endotoxin             No. of        No. of       Within-   Between-
Model(a)              homes       samples(b)      home       home

Bed dust
  A                     20           118          1.82       2.76
  B                                               1.80       2.89
Bedroom floor dust
  A                     20           200          1.71       1.53
  B                                               1.70       1.55
Kitchen floor dust
  A                     20           128          2.15       1.75
  B                                               2.15       1.81
Airborne endotoxin
  A                     15           139          2.30       1.52
  B                                               2.15       1.61

                     Variance
                      ratio        Within-
Endotoxin            (within/        home
Model(a)             between)   correlation(c)

Bed dust
  A                    0.35          0.74
  B                    0.31          0.76
Bedroom floor dust
  A                    1.61          0.38
  B                    1.48          0.40
Kitchen floor dust
  A                    1.88          0.35
  B                    1.65          0.38
Airborne endotoxin
  A                    3.93          0.20
  B                    2.56          0.28

(a) A: Variance components were estimated using a mixed-effect model
with a random home effect and fixed season effect; B: Variance
components were estimated using a mixed-effect model with a random
home effect and fixed season effect controlling for time-varying home
characteristics within home (operating humidifier and windows open)
for bed and bedroom floor dust endotoxin. For kitchen floor dust
endotoxin and bedroom airborne endotoxin, models additionally included
indoor climate parameters (wet- and dry-bulb temperature and absolute
and relative humidity). (b) There was a total of 446 dust samples with
dust available for the entoxin assay; 139 air samples were used in
data analysis. CRatio of between-home to sum of within-home and
between-home variance represents reproducibility of repeated
measurements within a home.


The overall crude GSD of airborne endotoxin was larger than that of TSP (Table 1). The large GSD for airborne endotoxin seemed to be driven by a large within-home variance component (Table 2 and Figure 4). The ratio of within-home to between-home variance of airborne endotoxin was larger than the ratio for any of the dust endotoxin levels.

[GRAPH OMITTED]

Correlation between endotoxin measurements. Mean bed endotoxin computed for each home was not significantly correlated with mean bedroom or kitchen floor endotoxin at ct = 0.05. Mean bedroom and kitchen floor endotoxin were significantly correlated (p [is less than] 0.05). Mean TSP and airborne endotoxin were not significantly correlated (Table 3).
Table 3. Spearman correlation coefficient between average(a) values
of airborne endotoxin, house dust endotoxin, and TSP concentration.

                                    House dust endotoxin (EU/mg)

Measurement               TSP      BB      BF      KF      Mean(b)

Airborne endotoxin(c)    0.13     -0.23   0.18    0.33      0.04
 [EU/[m.sup.3]]
                        (15)(d)   (11)    (14)    (14)      (15)
TSP (mg/[m.sup.3])        --      0.21    0.03    -0.24     0.26
                         (15)     (11)    (14)    (14)      (15)
Bedroom bed (EU/mg)       --       --     0.45    0.00       --
                                  (16)    (15)    (15)
Bedroom floor (EU/mg)     --       --      --    0.46(*)     --
                                          (19)    (19)
Kitchen floor (EU/mg)     --       --      --      --        --
                                                  (19)

Abbreviations: BB, bedroom bed; BF, bedroom floor; KF, kitchen floor.

(a) Average over all repeated measurements during the study period
by each sample type (BB, BF, KF, or air) within a home. (b) Mean
overall dust endotoxin measurements within a home during the study
period. (c) Airborne endotoxin was sampled in bedroom. (d) Number of
homes. (*) p [is less than] 0.05.


None of the mean dust endotoxin measurements (bed, bedroom, and kitchen floor) was significantly correlated with mean airborne endotoxin (Table 3). The relationship of dust and airborne endotoxin levels was further examined with a mixed-effect linear regression model, controlling for a random home effect and a fixed sampling month effect. However, we did not observe a significant association between dust and airborne endotoxin levels.

Endotoxin levels and home characteristics and climate parameters. In the crude analysis of correlations between average climate factors and average airborne endotoxin levels within homes, airborne endotoxin levels tended toward weak positive correlations Noun 1. positive correlation - a correlation in which large values of one variable are associated with large values of the other and small with small; the correlation coefficient is between 0 and +1
direct correlation
 with humidity (except for absolute humidity) and weak negative correlations Noun 1. negative correlation - a correlation in which large values of one variable are associated with small values of the other; the correlation coefficient is between 0 and -1
indirect correlation
 with temperature (except for wet-bulb temperature); none of these correlations was significant. Correlations of dust endotoxin with climatic factors showed similar patterns, as did airborne endotoxin (weakly positive with humidity and weakly negative with temperature). However, kitchen floor dust endotoxin was significantly correlated with bed and bedroom floor surface temperature at [Alpha] = 0.05 (Table 4). A total-home mean dust endotoxin level, calculated by averaging all types of dust endotoxin measurements within a home (bed, bedroom floor, and kitchen floor) was significantly and negatively associated with wet-bulb temperature.
Table 4. Spearman correlation coefficient (number of homes) between
average(a) values of airborne endotoxin and house dust endotoxin,
TSP concentration, and indoor climate.

                                                         House dust
                                                          endotoxin
Home climate                   Airborne(b)              level (EU/mg)
parameters                      endotoxin     TSP            BB

Percent relative humidity         0.18       -0.38           0.36
                                  (15)       (15)            (16)
Absolute humidity                -0.08      -0.39            0.31
                                  (15)       (15)            (16)
Water activity of bed             0.26       -0.48           0.18
                                  (15)       (15)            (16)
Water activity of BF              0.13       -0.43           0.35
                                  (15)       (15)            (16)
Dry-bulb temperature of air       -0.19      0.11            -0.48
                                  (15)       (15)            (16)
Wet-bulb temperature of air       0.08       -0.01           -0.09
                                  (15)       (15)            (16)
Surface temperature of bed        -0.21      0.28            -0.22
                                  (15)       (15)            (16)
Surface temperature of BF         -0.22      0.29            -0.31
                                  (15)       (15)            (16)

Home climate                     House dust endotoxin level (EU/mg)
parameters                         BF         KF             Mean(c)

Percent relative humidity         0.19       0.25            0.01
                                  (19)       (19)            (20)
Absolute humidity                 -0.03      0.03            -0.23
                                  (19)       (19)            (20)
Water activity of bed             0.31       0.32            -0.02
                                  (19)       (19)            (20)
Water activity of BF              0.31       0.42            0.10
                                  (19)       (19)            (20)
Dry-bulb temperature of air       -0.36      -0.41           -0.42
                                  (19)       (19)            (20)
Wet-bulb temperature of air       -0.22      -0.31           -0.59(*)
                                  (19)       (19)            (20)
Surface temperature of bed        -0.40      -0.51(*)        -0.28
                                  (19)       (19)            (20)
Surface temperature of BF         -0.34      -0.46(*)        -0.34
                                  (19)       (19)            (20)

Abbreviations: BB, bedroom bed; BF, bedroom floor; KF, kitchen floor.

(a) Average overall repeated measurements during the study period by
each sample type (BB, BF, KF, or air) within a home. (b) Airborne
endotoxin was sampled in the bedroom. (c) Mean overall dust endotoxin
measurements within a home during the study period. (*) p [is less
than] 0.05.


The association of endotoxin levels with home characteristics and indoor climate parameters was further examined with mixed-effect regression models that included a random home effect and a fixed sampling month effect. We found that certain home characteristics (wool bedding on bed, type of rug vacuumed in kitchen floor, foam pillow on bed, cotton bedding on bed, and operating humidifier) were significantly associated with airborne endotoxin. Of the climate parameters, only absolute humidity was positively [[Beta] = 0.5 (EU/[m.sup.3])/(10g [H.sub.2]O/kg air)] and significantly (p = 0.01) associated with airborne endotoxin levels. This association remained significant (p = 0.04) in a multivariate model after adjusting for the significant home characteristics.

In the mixed regression model controlling for the random home effect and the fixed effect of sampling month and sample type, none of the indoor climate parameters was significantly associated with dust endotoxin levels. We found that mattress type on bed, type of rug vacuumed in kitchen floor, type of rug vacuumed in bedroom floor, and operating humidifier were significantly associated with dust endotoxin in the same mixed models. The lack of association between climactic cli·mac·tic   also cli·mac·ti·cal
adj.
Relating to or constituting a climax.



cli·macti·cal·ly adv.

Adj. 1.
 parameters and dust endotoxin did not change after adjusting for the significant home characteristics.

Seasonal variation of indoor dust and airborne endotoxin levels. Our data did not suggest a consistent temporal pattern in endotoxin levels in settled dust. When we categorized sampling month by season, significant seasonal effects were observed in kitchen floor dust endotoxin levels; dust endotoxin level was highest in spring and lowest in fall. The seasonal effect on kitchen dust endotoxin remained significant (p [is less than] 0.01) in a multivariate model controlling for the time-varying home characteristics including home climate parameters (Table 5). However, we did not observe a seasonal influence on endotoxin levels in bed and bedroom floor dust.
Table 5. Seasonal variation in home endotoxin level--mixed-effect
regression models.

           Response var(b)                  Season: main effect
Model(a)   log (endotoxin)         No.   Variable(c)         p-Value

A          Bed dust                118   Season (F)           0.75
B                                        Season (F)           0.85
A          Bedroom floor dust      200   Season (F)           0.20
B                                        Season (F)           0.14
A          Kitchen floor dust(c)   128   Season (F)         < 0.01
B                                        Season (F)         < 0.01
A          Airborne endotoxin(c)   139   Season (F)           0.01
B                                        Season (F)           0.02

            Response var(b)
Model(a)    log (endotoxin)            Covariates(c)

A           Bed dust                     Home (R)
B                                    Home (R) + HC (F)
A           Bedroom floor dust           Home (R)
B                                    Home (R)+ HC (F)
A           Kitchen floor dust(c)        Home (R)
B                                    Home (R)+ HC (F)
A           Airborne endotoxin(c)        Home (R)
B                                    Home (R) + HC (F)

            Response var(b)           Least squares mean by season
Model(a)    log (endotoxin)         Spring    Summer   Fall    Winter

A           Bed dust                  40.3     46.2    48.9     42.0
B                                     40.7     46.7    45.0     37.8
A           Bedroom floor dust        83.5     68.0    67.3     71.2
B                                     86.5     73.9    66.4     69.3
A           Kitchen floor dust(c)    154.0     97.4    58.0     93.1
B                                    155.8     82.9    49.4    106.4
A           Airborne endotoxin(c)      0.94     0.59    0.77     0.51
B                                      0.98     0.50    0.64     0.60

Abbreviations: R, random effect covariate; F, fixed effect covariate;
HC, time-varying home characteristics within home. (a) A: mixed-effect
model with a random home effect and a fixed season effect (spring:
April-May, summer: June-August, fall: September-October, winter:
November-March). B: Mixed-effect model A is additionally adjusted for
time-varying home characteristics within home (windows open, number of
layers of bedding, and home climate parameters: humidity and
temperature), (b) Airborne endotoxin, EU/[m.sup.3]; dust endotoxin,
EU/mg. (c) Significant seasonal effect.


Our data showed a significant seasonal effect on airborne endotoxin levels (Table 5). Airborne endotoxin was highest in the spring and lowest in the winter, both before and after adjusting for time-varying home characteristics (Table 5). The only significant contrast between seasons, after adjusting for multiple comparisons, was between spring and winter.

Outdoor airborne endotoxin. Overall (crude) mean indoor airborne endotoxin levels (Table 1) appeared to be higher (GM = 0.64) than those in outdoor air (n = 70, GM = 0.46, GSD = 2.6). However, the mixed-effect regression model [including fixed effects for sampling site (indoor/outdoor), season, and a sampling site by season interaction, controlling for the random home effect] indicated that the seasonal effect and the sampling site by season interaction effects were both significant (p [is less than] 0.001 and p = 0.001, respectively). The airborne endotoxin level was not consistently higher indoors than outdoors (Figure 5). From September through April, indoor airborne endotoxin levels were generally higher than those outdoors; however, the difference was only significant during the winter (p = 0.01). Indoor levels tended to be somewhat but not significantly (p = 0.26) lower than outdoor levels during the summer (June through August).

[GRAPH OMITTED]

Analysis of the association between season and outdoor airborne endotoxin level, adjusted for multiple comparisons, indicated that the level during winter [least squares mean (LSM LSM Linux Software Map
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LSM Legato Storage Manager
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) = 0.19 EU/[m.sup.3]] was significantly lower than any other season (p [is less than] 0.05 for all pair-wise comparisons). The level was highest during summer (LSM = 0.92 EU/[m.sup.3]). Summer levels were significantly greater (p = 0.004) than levels during the fall (LSM = 0.42 EU/[m.sup.3]), but not significantly different from spring levels (LSM = 0.64 EU/[m.sup.3]).

Overall mean airborne endotoxin levels at the urban sampling location (n = 32, GM = 0.51 EU/[m.sup.3], GSD = 2.1) appeared to be higher than those at the suburban location (n = 35, GM = 0.39 EU/[m.sup.3], GSD = 3.1). However, the location effect was not significant in a regression model with a fixed location effect controlled for a fixed effect of season; we found no statistical evidence for an urban/suburban location by season interaction effect.

Discussion

Variance of and relationship between airborne and dust endotoxin. Rappaport and colleagues (11,12,20,21)and others (13,14) have used variance components (between- and within-person variance) to examine assumptions about homogeneity Homogeneity

The degree to which items are similar.
 of exposure within groups and the utility of exposure assessment strategies in occupational epidemiology. In this study we applied a similar approach to examine how well various measures of home endotoxin distinguish domestic exposure among a group of faculty, staff, and students at the Harvard School of Public Health. We found that the ratio of within-home to between-home variance was less than 1 for bed dust, but not for bedroom or kitchen floor dust endotoxin. The reproducibility of repeated endotoxin measurements within homes (within-home correlation coefficient) was greater for bed dust than for either bedroom or kitchen floor dust. This implies that if house dust is to be used for endotoxin exposure assessment, bed dust may provide better discrimination of exposure between individuals than can be achieved (24) with bedroom or kitchen floor dust. Our finding is consistent with the observation by Michel et al. (4) that endotoxin in dust collected from beds was significantly associated with asthma severity in house dust mite-sensitized adults. However, our data may also be consistent with the observation by Douwes et al. (24) that peak flow variability was not significantly associated with endotoxin in living room floor dust, and may help to explain these otherwise apparently discrepant dis·crep·ant  
adj.
Marked by discrepancy; disagreeing.



[Middle English discrepaunt, from Latin discrep
 results. The largest within-home variance for endotoxin in dust was observed in kitchen floor samples; water and organic material in the kitchen environment may provide more variable conditions for bacterial growth Bacterial growth

The processes of both the increase in number and the increase in mass of bacteria. Growth has three distinct aspects: biomass production, cell production, and cell survival.
 or accumulation of endotoxin than do the bed or bedroom floor.

The within-home geometric standard deviation ([GSD.sub.w]) and variance ratio were larger for airborne endotoxin, and the reproducibility was poorer than for house dust endotoxin (Table 2). Thus, merely on statistical grounds, it is clear that much greater effort would be required to discriminate domestic exposures using measurement of airborne rather than house dust endotoxin. This finding suggests that, rather than a single 24-hr air sample, multiple samples or a longer sampling period may be required to accurately assess airborne endotoxin exposure.

Whether dust endotoxin can be appropriately considered an indicator of endotoxin exposure, however, depends on some assumptions about how individuals are exposed. We found no significant association between dust endotoxin and airborne endotoxin in either crude correlation analyses or mixed regression models taking account of the repeated measures design of the study. Thus, dust endotoxin alone is a weak surrogate surrogate n. 1) a person acting on behalf of another or a substitute, including a woman who gives birth to a baby of a mother who is unable to carry the child. 2) a judge in some states (notably New York) responsible only for probates, estates, and adoptions.  for airborne endotoxin levels. If airborne endotoxin at home represents true exposure, then use of dust endotoxin as a surrogate will result in nondifferential misclassification of exposure. On the other hand, endotoxin in dust may be a significant direct source of exposure for infants, and endotoxin in the bed may be a significant source of exposure for both children and adults.

Therefore, two sources of bias toward the null A character that is all 0 bits. Also written as "NUL," it is the first character in the ASCII and EBCDIC data codes. In hex, it displays and prints as 00; in decimal, it may appear as a single zero in a chart of codes, but displays and prints as a blank space.  in analysis of exposure-response relationships should be considered when single measurements of house dust endotoxin are used for exposure assessment. The first source of bias arises if dust endotoxin is considered a surrogate measure for airborne endotoxin (25), and the second when the within-home variance is larger than the between-home variance (13). Bias due to these sources of error may be reduced through use of internal validation study designs.

The overall distribution (Table 1) of home endotoxin levels in this repeated measurement study of a small number of homes was comparable to the distribution we observed in a cohort study A cohort study is a form of longitudinal study used in medicine and social science. It is one type of study design.

In medicine, it is usually undertaken to obtain evidence to try to refute the existence of a suspected association between cause and disease; failure to refute
 involving 499 homes (data not shown), most visited only once. Therefore, the data used in this analysis appear to be representative, although due to the small number of homes, weak associations between air and dust endotoxin levels were not significant in the present study.

Endotoxin levels and temperature and humidity. Home airborne endotoxin may be more closely related to moisture (dampness) than to temperature. Simard et al. (26) reported that the level of bacteria measured inside the duct of an apartment building was associated with relative humidity in the duct. In numerous studies (27-31), home dampness has been significantly associated with children's respiratory disease and symptoms. Our findings suggest that home endotoxin may be among the exposures responsible for the association, and may be a reasonable objective measure of the biological burden resulting from dampness.

Seasonal variability of home and outdoor airborne endotoxin. Little data is available on seasonal variation of endotoxin in homes. Rizzo et al. (3) reported seasonal variation in house dust endotoxin with significantly lower endotoxin levels during the winter in 20 homes sampled 13 times during a 1-year study in Brazil. Our data from Massachusetts showed significant seasonal variation in kitchen dust and airborne endotoxin levels, but not in bed and bedroom dust endotoxin. The air and floor dust samples had their highest endotoxin levels in the spring, whereas endotoxin in bed dust was relatively constant. However, the range of mean endotoxin variation across seasons was small, [is less than or equal to] 2-fold for all samples except for the kitchen floor. Thus, evidence to suggest an important seasonal pattern in home endotoxin is weak.

On the other hand, we observed a significant seasonal pattern in outdoor airborne endotoxin level; mean outdoor endotoxin levels varied by more than a factor of four across seasons. There was a decline of outdoor airborne endotoxin beginning at the end of summer or early in the fall. Outdoor endotoxin remained low during the winter and started to increase with the beginning of growing season growing season, period during which plant growth takes place. In temperate climates the growing season is limited by seasonal changes in temperature and is defined as the period between the last killing frost of spring and the first killing frost of autumn, at which . Our observations are consistent with the data suggesting that outdoor gram-negative bacteria, and thus airborne endotoxin, are shed from leaves of growing plants (32,33). The difference between indoor and outdoor airborne endotoxin levels varied with season. Our data indicate that indoor airborne endotoxin levels are significantly higher than outdoor levels during the winter, but similar to outdoors during the spring, summer, and fall. Thus, while indoor sources clearly predominate during the winter, outdoor airborne endotoxin may contribute to indoor airborne endotoxin, especially during the spring, summer, and fall when endotoxin levels indoors are relatively constant and homes are not tightly sealed.

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pertaining to or emanating from a microbe.


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(2.) Milton DK, Johnson DK, Park JH. Environmental endotoxin measurement: interference and sources of variation in the Limulus assay of house dust. Am Ind Hyg Assoc J 58:861-867 (1997).

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JSM John Sidney McCain
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Of, relating to, containing, or derived from phenol.

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Any of various synthetic thermosetting resins, obtained by the reaction of phenols with simple aldehydes and used as adhesives.
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cracklings, an edible raw fat from the meat trade. The skimmings from the preparation of this fat are also called greaves. They represent a low grade of meat meal.
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Relating to the bronchi, the bronchial tubes, or the bronchioles.
 obstruction in young Norwegian children. Am J Respir Crit Care Med 157:410-414 (1998).

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Of, relating to, or being the geographic areas adjacent to the Tropics.


subtropical
Adjective

of the region lying between the tropics and temperate lands

 climate. Environ Res 75:49-55 (1997).

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n an investigation employing an epidemiologic approach in which previously existing incidents of a medical condition are used in lieu of gathering new information from a randomized population.
. Thorax thorax, body division found in certain animals. In humans and other mammals it lies between the neck and abdomen and is also called the chest. The skeletal frame of the thorax is formed by the sternum (breastbone) and ribs in front and the dorsal vertebrae in back.  52:229-234 (1997).

(31.) Spengler J, Neas L, Nakai S Nakai may refer to: People named Nakai
  • Kazuya Nakai, actor
  • Kiichi Nakai, actor
  • Masahiro Nakai, musician
  • Nakai Chikuzan, politician
  • R. Carlos Nakai, musician
  • Takenoshin Nakai, botanist
, Dockery D, Speizer F, Ware J, Raizenne M. Respiratory symptoms and housing characteristics. Indoor Air 4:72-82 (1994).

(32.) Edmonds RL, ed. Aerobiology aerobiology /aero·bi·ol·o·gy/ (ar?o-bi-ol´o-je) the study of the distribution of microorganisms by the air.

aer·o·bi·ol·o·gy
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, The Ecological Systems Approach. Stroudsburg, PA:Dowden, Hutchinson & Ross, 1979.

(33.) Andrews JH, Hirano SS, eds. Microbial Ecology Microbial ecology

The study of interrelationships between microorganisms and their living and nonliving environments. Microbial populations are able to tolerate and to grow under varying environmental conditions, including habitats with extreme environmental
 of Leaves. New York New York, state, United States
New York, Middle Atlantic state of the United States. It is bordered by Vermont, Massachusetts, Connecticut, and the Atlantic Ocean (E), New Jersey and Pennsylvania (S), Lakes Erie and Ontario and the Canadian province of
:Springer-Verlag, 1992.

Ju-Hyeong Park,(1) Donna L. Spiegelman,(2) Harriet A. Burge,(1) Diane R. Gold,(3) Ginger L. Chew,(4) and Donald K. Milton(1)

(1) Department of Environmental Health, and (2) Departments of Epidemiology and Biostatistics, Harvard School of Public Health, Boston, Massachusetts “Boston” redirects here. For other uses, see Boston (disambiguation).
Boston is the capital and most populous city of Massachusetts.[3] The largest city in New England, Boston is considered the unofficial economic and cultural center of the entire New
, USA; (3) The Channing Laboratory, Department of Medicine, Brigham and Women's Hospital Brigham and Women's Hospital (BWH) is a hospital in the Longwood Area of the Boston, Massachusetts neighborhood of Mission Hill. With Massachusetts General Hospital, it is one of the two founding members of Partners HealthCare.  and Harvard Medical School Harvard Medical School (HMS) is one of the graduate schools of Harvard University. It is a prestigious American medical school located in the Longwood Medical Area of the Mission Hill neighborhood of Boston, Massachusetts. , Boston, Massachusetts, USA; (4) Division of Environmental Health Sciences, J.L. Mailman School of Public Health, Columbia University Columbia University, mainly in New York City; founded 1754 as King's College by grant of King George II; first college in New York City, fifth oldest in the United States; one of the eight Ivy League institutions. , New York, New York, USA

Address correspondence to D.K. Milton, Environmental Health, Occupational Health Program, Harvard School of Public Health, 665 Huntington Avenue, Boston, MA 02115 USA. Telephone: (617) 432-3324. Fax: (617) 432-0219. E-mail: dmilton@hohp.harvard.edu

We thank K. McGaffigan for assistance with data management and analysis, the research assistants who assayed samples, and especially the study participants who answered questionnaires and collected dust and air samples.

This study was supported by National Institute of Environmental Health Sciences The National Institute of Environmental Health Sciences (NIEHS) is one of 27 Institutes and Centers of the National Institutes of Health (NIH),which is a component of the Department of Health and Human Services (DHHS). The Director of the NIEHS is Dr. David A. Schwartz.  grant R01 ES-07036, National Institute of Environmental Health Sciences Center grant 2P30ES00002, and a gift from Bio Whittaker, Walkersville, MD. J.-H. Park received a Korea Industrial Safety Corporation Scholarship.

Received 13 March 2000; accepted 28 June 2000.
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