Ambient particulate matter air pollution (PM) is one of the top ten causes of illness and death in the world. While PM pollution is worse in many developing countries, it remains a problem in the United States as well. Many people in the US may not be aware of the magnitude of the problem because the levels of pollution that present a health risk include concentrations that are usually not readily visible.
PM that is less than 2.5 microns in diameter, called PM2.5, is the form of PM that is usually considered when assessing health risks. It is also regulated at the federal level in the US. Because of the deep research base and government regulation that follows from the evidence, PM2.5, also called “fine PM,” gets some well-deserved attention. However, there is rapidly emerging evidence that another type of PM, ultrafine particles, represents an additional distinct and unregulated risk.
Ultrafine particles (UFP) differ from PM2.5 in that they are much smaller (<0.1 microns). Because they are so small, they have very little mass. Thus, whereas PM2.5 is measured by weight, UFP are usually measured by counting the number of particles in a volume of air, typically a cubic centimeter. However, their small size is not the only thing that distinguishes UFP from PM2.5. They also behave differently in the air and are elevated in different patterns in time and across geographic areas.
I have been involved with research that is among a growing number of teams that have assessed UFP distribution in urban areas. Because cars are the dominant source of UFP in most urban areas in countries like the US, we have focused on major roadways and highways. There is, and has been for a while, convincing evidence that UFP levels are substantially elevated next to such roadways on a regular basis.
In contrast, PM2.5 has much less dramatic levels immediately adjacent to traffic sources. This is because PM2.5 is formed largely from precursor gasses by chemical processes that take time to evolve. PM2.5 is often found to spread over 10s or 100s of kilometers with some variation in levels, but not nearly as dramatic as those found for UFP.
From a health research standpoint, this has made it much easier to study PM2.5 than it is to study the health effects of UFP. People living in a single metropolitan area, for example, will usually have similar PM2.5 exposures. The same people could, however, have very different UFP exposures, especially if they differ in terms of living or spending time next to a highway. Just 100 meters away from high volume traffic UFP levels are substantially lower.
Much of the challenge of the UFP health research I have been a part of has had to grapple with the fact that their concentration changes rapidly in both space and time while people move into and out of fields of exposure. The excruciating care required to assign exposure has held back health research on UFP. It is only in the last 3-4 years that numerous studies showing risks from long term exposure have begun to emerge. So far findings point particularly at cardiovascular and neurological risks.
Because they are so small, UFP act more like gases than particles in the air. Like gasses, they tend to diffuse rather than the way PM2.5 settle or impact on surfaces. This affects strategies to reduce exposure. My collaborators and I like high-quality filters that remove UFP effectively for use in near-highway homes. We also prefer substantial air recirculation since it reduces infiltration into indoor spaces. Air movement also leads to great deposition from diffusion.
While the US Environmental Protection Agency is not yet convinced there is a causal link between UFP and health, they have been paying close attention and devoted considerable space to UFP in their recent Integrated Science Assessment for PM. Until the research reaches a critical mass and regulations are enacted, we need to continue doing research, educating the public and policy makers about this concern and developing and implementing local interventions to proactively protect highly exposed populations.