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This essay is in response to: What is the most important achievement we've gained through air pollution management? What remains to be done to safeguard public health from air pollution?
In the 1990s there was a shocking discovery. After more than 20 years of federal regulations aimed at reducing negative impacts of air pollution and clearing skies above American cities, two major studies found that the pollution that remained was still strongly associated with mortality. The Harvard Six Cities Study and the American Cancer Society Study were large, prospective investigations that showed that fine particulate pollution (technically, PM2.5, or particles with a diameter less than 2.5 microns) was causing respiratory and cardiovascular diseases and death. Following these studies the US EPA began regulation of PM2.5.
Yet, today, the most recent estimates still suggest that more than 100,000 Americans die each year, mostly from cardiovascular disease, from breathing in the (reduced) levels of PM2.5 that remain in our air. Indeed, PM2.5 appears to have health consequences similar to the effects of second hand smoke, an exposure that the public will no longer tolerate.
Most Americans are unaware that particulate pollution is the single most deadly pollution they face (and the pollutant of greatest economic consequence). Nor is there much awareness that existing regulations are inadequate. EPA is likely to propose lowering the PM2.5 standard modestly as part of a legally mandated review, a reduction that would save lives, but not eliminate the hazard. Despite the cautious nature of this proposal, EPA is under attack for “killing jobs” rather than lauded for trying to saving lives.
Unfortunately, PM2.5 is not the only particulate pollution of concern. Credible city-wide studies by respected environmental health scientists in Stockholm and Oslo have found elevated lung cancer mortality risk among those most exposed to vehicular pollution at their residential addresses a decade or two earlier. Similarly, North American scientists have found significantly elevated cardiovascular mortality risk for residents living near highways in Vancouver and for those with pre-existing respiratory disease in Toronto who also have high levels of vehicular pollution at their residential addresses.
These “near source” health impacts may be a result of differential exposure to particles less than 100 billionths of a meter in diameter. It seems that the smaller tiny airborne particles are, the more toxic they become. PM2.5 is a regional pollutant that is spread out over, for example, metropolitan areas. In contrast, the smallest particles, called ultrafine particles (UFP), are elevated more locally, typically next to highways and major roadways. UFP also change in time more rapidly than do PM2.5, so measuring them is a more challenging exercise, perhaps a reason that they have so belatedly begun to attract attention.
When motor vehicles are running, especially when they stop and start in rush hour congestion, they emit hot gasses from their tailpipes. The hot gasses cool very quickly in the air and many condense into particles. The result is UFP that are molecular in scale and are distant cousins of manufactured nanoparticles. These UFP are so small that when we monitor them we usually just count their number, because they have so little mass.
There are now dozens of studies, including one that we helped author, that have measured UFP next to highways. The results are largely consistent across studies. UFP are highest next to the highway on the downwind side and drop off rapidly (exponentially) to urban background levels within a few hundred meters. You might think that this means few people are exposed, but you would be wrong. More than 10% of Americans live that close to highways and more live near major roadways. Many more drive regularly on these highways, breathing in millions of UFP in the cab of their vehicle.
But are UFP a serious health hazard? While the causal link is not fully closed from a scientific perspective, there is an awful lot of smoke for there to be no fire. A series of epidemiology studies suggest they are. These studies have shown associations of proximity to highways, major roadways, high density of traffic, and models of markers of traffic-related air pollution with both respiratory and cardiovascular effects.
Second, studies from California and Europe have shown strong associations between UFP and markers of cardiovascular health in elderly people. Third, toxicological studies have found that UFP cause cardiovascular effects in exposed animals. Finally, although few in number, there are studies beginning to look at whether reducing UFP exposure (through air filtration) leads to improved markers of cardiovascular health.
Thus, UFP, which are not regulated by the EPA, almost certainly add to the toll already documented from PM2.5. Today there is no national network of monitors to track UFP levels, although the EPA is preparing an exploratory network of monitors. These monitors might begin to collect regular data on the level of the problem. We think that federal regulation is needed, but we understand that mitigation will have to start in advance of EPA taking action, especially in the present anti-regulatory climate.
One example of forward thinking is that the State of California has restricted construction of new schools next to highways. But enforcement of this rule is weak. HEPA filters are also being installed in existing schools near the Ports of Los Angeles and Long Beach, following a successful pilot project. In the Boston area, we have begun exploring the value of in-home air filtration in two pilot research studies, but results are a couple of years away. Other possibilities include “noise barriers,” use of central air conditioning, keeping windows closed and avoiding outdoor exercise during peak UFP times, and preventive medication for high risk individuals living near highways. All of these appear to us to have potential to either reduce exposure or reduce risk of adverse health outcomes. But more research is needed, along with education of the public and policy makers, as well as health care providers, who are largely unaware of the problem.
The authors are partners on the NIH-funded Community Assessment of Freeway Exposure and Health study.
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A very interesting and thought-provoking article. I'd like to hear more about this study being done.
April 14, 2011