What are you breathing?

The oldest continuously operated air sampling station in Massachusetts is not much to look at, and for more than 30 years, Boston University students have walked through Kenmore Square and looked past its nondescript trailers, which are just now yielding startling evidence about potentially dangerous particles in the air on the fringes of the BU campus.

The most recent data available from Kenmore Square show that average concentrations of four pollutants monitored there since the 1980s, despite heavy vehicle traffic on Beacon Street, Commonwealth Avenue, and the nearby Massachusetts Turnpike, have remained better than federal standards — but that may not mean that a clean bill of health for the square. Scientists have recently started measuring a new class of particles that could cause concern.

Monitoring for very small particles suspended in the air, known as PM2.5 (particulate matter smaller than 2.5 microns in average diameter) began at Kenmore Square only in 2000. Data from 2000 and 2001 suggest that all the traffic near the square, including the nearly 40 million vehicles that pass by Boston University each year on the Massachusetts Turnpike, has made the airborne concentration of PM2.5 there the highest of any of the twenty-nine sampled sites in the state.

Particulate matter consists of a suspended soup of small solids from a variety of sources. They include dust, molds, and soot, and can cause eye, nose and throat irritation, among other health problems.

‘Most reasonably healthy folks don’t need to be concerned too much,’ said George Allen of the Northeast States for Coordinated Air Use Management, an association of air quality control divisions in eight Northeast states.

Noting the considerable uncertainty on the topic, he added, ‘There are probably significant long-term effects of exposure to these [PM2.5] levels. Those with pre-existing cardiac or pulmonary disease are the ones most at risk in the short term.’

Twenty-two of the state’s 29 monitoring sites for this pollutant, including the monitoring station at Kenmore Square, failed in more than 25 percent of their sampling efforts in at least one quarter of 2001, according to the Commonwealth of Massachusetts Air Quality Report 2001.

Such high failure rates make interpretation and comparison of the data problematic, but what data is available suggests that Kenmore Square, if the values produced from the problem-plagued monitoring are indicative, is one of only three monitored sites in the state headed towards violating federal air quality standards for PM2.5.

Three years of satisfactory data are required to determine whether a site is in compliance with federal standards, and Allen said it is not clear how the EPA will assess the incomplete PM2.5 data collected in 2000 and 2001.

Concentrations of PM2.5 have only been widely measured in the United States for about the past five years, according to David Conroy of the federal Environmental Protection Agency. Massachusetts began monitoring PM2.5 in 1999, and the Kenmore Square site started measuring PM2.5 only in 2000, according to the report.

The average Kenmore Square value calculated for the year 2000 was about 90 percent of the federal standard, according to a similar report in 2000. The same measure for 2001 exceeded the federal standard. The 2001 report says that the percentage of unusable samples in the PM2.5 monitoring network as a whole declined from 2000 to 2001-to 41 percent.

PM2.5 are among the pollutants of greatest concern at the moment in New England, Allen said, because there ‘is no threshold’ concentration known at which the pollutant does not cause damage to health. These particles are so small that several thousand could fit on the period at the end of this sentence, according to the report

The Kenmore Square monitoring station also measures several pollutants other than PM2.5, but what is measured has changed over time, and only some pollutants speak to long-term trends.

For the four pollutants monitored at the site since the 1980s – PM10 (particulate matter 10 microns and smaller in average diameter) and the gases carbon monoxide, nitrogen dioxide and sulfur dioxide – the data in the 2001 report show that concentrations have declined considerably, though at different rates, and not always consistently.

‘The things we monitor there have dropped a lot,’ says Tom McGrath of the Massachusetts Department of Environmental Protection, which runs the Kenmore Square monitoring site.

Though consistently far below federal standards, the concentrations of PM10 and nitrogen dioxide increased slightly last year, while the concentrations of carbon monoxide and sulfur dioxide decreased slightly, according to the 2001 report.

‘Back in the 1970s, power plants and other sources had no controls, and belched out black smoke,’ explained EPA’s Conroy. Boston residents once contended with visible, grimy soot on cars and buildings, he said.

The average concentration for PM10 was nearly twice as low as the federal standard required in 2001, though up slightly from 2000, and down about 15 percent from 1989, the earliest year presented in the 2001 report.

PM10 poses similar health risks as PM2.5, but because the particles tend to larger, the particles penetrate the lungs less deeply and the health effects are considered less damaging.

Conroy says pinpointing the reasons for this trend in particulate matter is more difficult than for the gaseous pollutants measured at the site, but he guessed cleaner-diesel vehicles and controls for emissions on fuel-burning power plants had contributed.

Incomplete combustion of fuels like gasoline produced the bulk of carbon monoxide registered by the state’s monitoring network. Breathing the gas at high enough concentrations reduces oxygen in the blood and can cause shortness of breath and loss of coordination. It poses particular dangers to those with heart and circulatory problems.

By convention, the 2001 report presents the concentration of only the second-highest eight-hour period each year, and that value has decreased two-thirds since 1985. The 2001 figure, down slightly from 2000, was only about one-quarter the federal standard. Improvements to automobiles, including catalytic converters and fuel-injection systems, have caused most of that decrease, said Bryan Glascock of the city’s Air Pollution Control Commission.

Coal-burning power plants and industrial processes such as smelting emit most of the sulfur dioxide measured. This gas can combine with water in the air to form chemicals that can harm lungs and cause problems for people with asthma or bronchitis. The 2001 report shows that the average value for that year, slightly down from 2000, was less than one-fifth the federal standard, and less than half its 1985 level.

State and a national acid rain programs over the past twenty years have reduced the sulfur in coal, oil and diesel fuel, according to Conroy, who says the long-term decrease at Kenmore is in line with trends for monitoring stations across the Northeast.

The primary sources of nitrogen oxides, including nitrogen dioxide, are motor vehicles, electric utilities, and other sources that combust fuel. Nitrogen dioxide can react to form many different chemicals, including particulate matter. The report shows that between 1985 and 2001, the annual average concentration of nitrogen dioxide at Kenmore Square decreased about a quarter, though the number has not changed much since 1995. In 2001, the figure was 40 percent below the federal standard, up slightly from 2000.

The decrease in nitrogen oxides during the 1980s came from reductions from car and truck emissions, followed by reductions from power plants in the early 1990s, according to Conroy. Emissions from increasingly popular SUVs and minivans, he added, have contributed significant amounts of nitrogen oxides in the last few years, as has increased traffic volume.

Even annual average concentrations of pollutants below federal standards do not mean that there are no health risks from the measured pollutants, warned NESCAUM’s Allen. He said the averaging of recorded values over a year could mask times when there are dangerous concentrations of some pollutants.

The data collected at Kenmore Square is used to determine whether an area is meeting standards, to refine strategies for improving air quality, and to inform the public of bad air-quality days, said Conroy. Conroy explained that real-time distribution of information is currently only done for ground-level ozone, which is measured in the Massachusetts network but not at the Kenmore Square site. He said that information on concentrations of particulate matter from the Kenmore site would be distributed in the near future.