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NAAQS [PM.sub.2.5] or ISO 14644 As an IAQ Metric?

A NAAQS is more than just a concentration--it has an indicator, a level, an averaging time and a form--and requires compliance with of all of these factors; it's not just a numerical level. For example, the [PM.sub.2.5] NAAQS has two averaging times (daily: 24-hour average, annual: yearly average of daily), a form (the three-year average of yearly 98th percentile daily values), and different concentration level for each of its two averaging times.

EPA also specifies type and siting of compliance monitors. [PM.sub.2.5] federal reference monitors and federal equivalent monitors are designed and configured to provide an accurate gravimetric measurement of the [PM.sub.2.5] fraction. This equipment is large (e.g., 2 m to 15 m minimum inlet height requirement) and generally unsuitable for indoor use (see Photo 1) to assess building occupant exposures, because siting requirements cannot be met, and would not represent indoor occupant exposures. EPA compliance monitor siting is selected to capture a sample to represent community exposures, without being impacted by surrounding structures or specific nearby sources.

Similarly, when particulates are assessed in the workplace, OSHA particulate standards or guidelines are either size specific, with a stated gravimetric or impinger sample method (such as for asbestos, cotton dust, or mineral dusts), or involve multiple size fractions in the case of particulates not otherwise regulated (PNOR) (1) without specifying the gravimetric method to be used.

There are primary, health-based NAAQS, and secondary welfare-based NAAQS. Epidemiological studies of large populations have relied on gravimetric air quality compliance monitoring to investigate associations with health end points. The historic gravimetric data and the multicommunity health studies based on those data form the foundation of primary PM NAAQS. They are not based on studies of small groups of people, or small areas within a community, and not on specific building occupants.

Sometimes real-time, indoor, nephelometric particulate measurements have been compared to the U.S. EPA primary PM NAAQS or to OSHA standards, despite deviations from the sampling requirements of those standards. Nephelometers are portable, cost less than reference method monitors, and can easily be deployed indoors. They are helpful if the user is mindful of the technology's limitation.

Nephelometers detect particles, then measure light scattering by the particle, and using assumptions about optical properties and density, assign a size to each particle to produce a mass concentration estimate--not a mass measurement. If the instrument has been calibrated using the very same particulate it is measuring in the field, then the default assumptions are valid and the mass estimate may closely approximate a gravimetric result. This is almost never the case.

The readings from a nephelometer are at best an estimate, and from comparison studies of factory-calibrated nephelometers with co-located gravimetric instruments, usually an overestimate of mass. Add to that the episodic, short sampling time for most building investigations, and it's pretty clear that nephelometer results bear little resemblance to air quality compliance monitoring that was the basis for setting health-based NAAQS standards.

So why is indoor particulate measurement useful? Real-time, multi-fraction PM monitoring is an important addition to conventional IAQ sampling. Low-cost particle counters can simultaneously count and log multiple particle fractions. Focusing on count, rather than mass concentration extrapolated from a count, removes the error introduced with default assumptions about the optical properties and density of the airborne particulates present. Logged particle data can identify events attributed to occupants versus events related to ventilation systems that occur when the IAQ professional isn't present and when occupant logs are incomplete. To be truly useful though, it's important to understand different sources of different particle fractions.

IEQ professionals are accustomed to using carbon dioxide levels as an indicator of occupancy and the adequacy of ventilation, although C[O.sub.2] concentrations are not necessarily good indicators of these parameters. Logging coarse (between 10 and 2.5 micrometers in diameter) and fine (2.5 to roughly 0.5 micrometers) particles along with gaseous and volatile components can refine building assessments. C[O.sub.2] levels gradually rise as occupancy increases and is maintained. Coarse particle counts rise rapidly when people arrive at work, take coats off, and decline as everyone settles in. Fine particles don't show nearly that dramatic a rise--but if someone burns toast--the fine particle count climbs.

The evidence for health effects from ambient PM exposure is robust and consistent. There is no identified level of PM exposure below which health effects are not observed. Lower is better; adverse health effects are associated with incremental increases over background, rather than with an absolute concentration. PM indicators have been refined over the years to target measurement of the fraction most strongly associated with observed health effects. While there is overlap in effects, fine particles are most closely associated with hospitalizations and deaths from cardiovascular diseases, and coarse fractions are more closely associated with hospitalizations and mortality from respiratory diseases. (2) The role of ultrafine particles (particles 100 nanometers or less in size) in health will be better understood as technologies to measure this fraction are deployed and integrated with health data.

In light of the above discussion on the limitations of applying the PM values from NAAQS indoors, and the necessary assumptions (often inaccurate) to convert PM counts to mass units, there is value in using count concentrations and aiming for "lower is better" or a percent reduction rather than a fixed target when addressing indoor PM. Nephelometers can provide rapid results at multiple locations, or multiple time points in the time required to collect a single typical gravimetric sample- perhaps by as much as 30 to one.

We have not located any health-based generic PM limits using real-time count concentration values (certain workplace mineral dust standards require collection with an impinger, followed by microscopic counts). As discussed above though, there are no recognized threshold values for PM health effects; incremental increases impart additional risk. This suggests that a quantitative scale, using measurements obtained in a consistent manner, can evaluate the indoor PM load. This evaluation could compare spaces to outdoors or assess a given space over time and provide practical, inexpensive guidance on managing the PM burden in a space.

An approach to using PM counts to quantify the PM burden is to rate the cleanliness class of a space using the method outlined in ISO 14644-1:2015. (3) This standard evaluates the cleanliness of spaces based on PM count concentration and provides a class designation (N) from 1 (lower concentration) to 9 (higher). This standard is focused on the cleanroom industry. Cleanrooms often are rated at Class 4 or lower, but as shown in Figure 1, many typical indoor environments fall into the higher end of the cleanroom scale, Class 8 and Class 9. These would not be considered "cleanrooms" for manufacturing microchips or pharmaceuticals, but they do not need to be; they need to provide comfortable and healthy environments for people.

Measuring PM levels by count concentration has several advantages for indoor environment assessments:

* The data collection can be conducted with readily available instruments that are widely used and familiar to many professionals in the field.

* There is no need for data conversion (based on invalid assumptions).

* There is an accepted standard method for data reduction into a single easily understood metric.

* The observed values overlay the range seen in "normal" occupancy situations (7 is great, 8 is good, 9 is normal for many nonindustrial spaces).

A suggestion is to use count concentrations for indoor assessment of PM and to leave the NAAQS outdoors.

After all, it was never intended to be an indoor creature. The health effects associations that can be made for PM mass measurements, in most cases, are likely to exhibit similar trends with PM count measurements. Further, there are established procedures, available instruments, and familiarity in the IAQ field with real-time particle counts. Most importantly, a focus on count concentrations for indoor PM levels shifts the emphasis to lowering the PM levels, regardless of baseline, rather than aiming for an indoor target level, which has not yet been established for health effects.


(1.) OSHA. 2017. "29 CFR 1910.1000," Tables Z-1 and Z-3.

(2.) EPA. 2009. "Integrated Science Assessment for Particulate Matter," pp. 2-9 to 2-12. EPA/600/R-08/139F. U.S. Environmental Protection Agency.

(3.) ISO 14644-1:2015, Cleanrooms andAssociated Controlled Environments--Part 1: Classification of Air Cleanliness by Particle Concentration.

(4.) Horner, E. N. Sanders N. 2015. "Application of ISO 14644 as an Indoor Air Quality (IAQ) Metric." American Association for Aerosol Research, Annual Conference Paper Number: 2IA.2.


Patricia Mason Fritz is a research scientist in the Center for Environmental Health at the New York State Department of Health. Elliott Horner, Ph.D., is lead scientist with UL Environment in Marietta, Ga. He is the incoming vice chair of ASH RAE's Environmental Health Committee and a member of SSPC 62.1, Ventilation and Acceptable IndoorAir Quality in Residential Buildings, and several other committees.

RELATED ARTICLE: PM Sampling Case Study.

This is a case study of PM sampling in two office suites, one with air quality complaints and the other without.

The building operated on the same ventilation program 24/7. On workdays, all areas logged increases in C[O.sub.2], but persistent C[O.sub.2] levels more than 700 ppm above outdoor concentrations were recorded only in the complaint suite.

Relying solely on the logged C[O.sub.2] data we collected, we might have concluded that it was an office crowding or low ventilation rate issue. However, a seemingly quirky rise in fine particle counts on the weekend (when the office was unoccupied), that was most noticeable in the non-complaint office suite, prompted us to reconsider.

On the weekend, C[O.sub.2] levels remained at background levels everywhere. The monitored areas logged no change in coarse particle counts, but rising fine particle counts, with a greater increase in the non-complaint areas. The increase in fine particle counts indoors roughly tracked rising fine PM mass measured at a nearby ambient air compliance monitor. Fine particle counts rapidly declined indoors along with fine particle mass at the compliance monitor after a weekend rainstorm. Lower C[O.sub.2] levels in the non-complaint suite, combined with a greater impact from outdoor pollution, supported a conclusion that the non-complaint area was likely receiving more outdoor air (the use of "fresh air" could be misleading in this scenario) than the complaint area.

It wasn't particle mass, or particle numbers, but rather relative changes in counts indoors and mass outdoors that informed our investigation.

Caption: PHOTO 1 Compliance monitors for NAAQS PM.

Caption: FIGURE 1 Distribution of ISO Class results for 82 indoor locations assessed by particle count concentration (5 micron). (4)
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Author:Fritz, Patricia Mason; Horner, Elliott
Publication:ASHRAE Journal
Article Type:Column
Geographic Code:1USA
Date:Jun 1, 2017
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