An ounce of pollution: particles' harm varies by person, region, season.

A gram of small, air-polluting particles has deadlier effects in certain seasons and regions of the country than in others, new research shows. Furthermore, particulate pollutants and ozone seem to most readily affect people who already have cardiovascular problems.

Many studies have shown that minute particles in air pollution have harmful effects over exposure periods both short (SN: 8/2/03, p. 72) and long (SN: 9/11/04, p. 163). Since air pollution consists of many types of particles as well as of various gases, researchers are investigating which pollutants are worst and whom they are most likely to hurt.

To document short-term regional and seasonal differences in pollution-related deaths, biostatistician Roger D. Peng and other researchers at Johns Hopkins University in Baltimore compiled data from the 100 largest U.S. cities. For each day from 1987 through 2000, they noted deaths and concentrations of airborne particulate matter less than 10 micrometers across, which is designated as PM10.

Year-round and nationwide, an increase of 10 micrograms of PM10 per cubic meter ([micro]g/[m.sup.3]) of air boosts deaths on the following day by an average of 0.19 percent, Peng and his colleagues report in the March 15 American Journal of Epidemiology. Typical variations in weather, traffic, and other factors can also produce day-to-day shifts of that magnitude, Peng says.

When the researchers analyzed data by region and season, they found marked differences. In the Northeast, for example, mortality rose in midsummer by almost 1 percent on any day after a 10 [micro]g/[m.sup.3] increase in PM10. The same increase in particulates boosted deaths by just 0.1 percent in midwinter.

By contrast, Peng says, "in southern California, there's very little seasonal variability." A 10 [micro]g/[m.sup.3] rise in particulates increases deaths by about 0.5 percent year-round.

These differences may reflect seasonal changes in the mix of chemicals that make up PM10 and changes in how much time people spend outside, the researchers say.

In a separate study, Sung Kyun Park of the Harvard School of Public Health in Boston and his collaborators there and at Boston University considered heart and circulatory problems and medication use among 497 local men of average age 73. The team also measured how much each volunteer's resting heart rate naturally varied from one beat to the next. Past research has linked low heart-rate variability to cardiovascular problems.

At the time of each exam, the researchers also measured several forms of particulate air pollution and gases at a site near the medical center. In the March Environmental Health Perspectives, they report an association between low heart-rate variability and high concentrations of particles 2.5 [micro]m or less across. High concentrations of ozone are also associated with low heart-rate variability, they find.

Men with ischemic heart disease or hypertension showed the greatest effect from exposure to those pollutants, the researchers report. "People with hypertension or heart disease need to be protected from the effect of air pollution" says Park.

Certain cardiovascular medications--beta-blockers and calcium-channel blockers--appear to counteract some of the threat posed by the air pollution.

Each pollutant reduced a different aspect of heart-rate variability, Park says.

The findings could lead to better recommendations for alerting people to dangerous pollution conditions, the scientists say.