Near Highway Pollutants
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Near Highway Pollutants in Motor Vehicle Exhaust
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Near Highway Pollutants
- 1. Near Highway Pollutants in Motor Vehicle Exhaust Michelle Hofmann, MD, MPH
- 3. Background 11% of US households are located within 100 meters of 4 lane highways Near highway pollutants may pose greater health risks than ambient air pollutants
- 13. KEY POINT: Proximity is Key Much higher exposures to traffic-related air pollutants occur within 30 meters as compared to >200 meters
- 15. KEY POINT: We Don’t Need to Measure Everything Because of their common source—vehicle emissions—UFP, NOx, BC and CO are highly correlated
- 18. Questions?
Notas del editor
- When I started preparing for this talk and looked more closely at the health effects literature that specifically targeted near roadway air pollutants it quickly became clear to me that this is a very complicated science. Though I am very familiar with the health effects of PM2.5, ozone, even diesel exhaust, I was surprised to learn that that study of near roadway pollutants is quite a bit different than that of the regional criteria air pollutants that I have been trying to get a better handle on for the last few years.
- The pollutants of concern near roadways are different than the criteria pollutants that Bryce talked about last time as are the challenges in monitoring them. So before I talk about health effects today, what I would like to do first is describe some of these differences so as to put in better perspective the health effects when I summarize them and give us a better sense of the kind of air monitoring project that might lie ahead for us.
- The exposure of near-highway residents to pollutants in automobile exhaust has only recently been studied. As Bryce helped explain last time, air monitoring programs are typically set up to measure regional not local air quality. As you will see as we go further in this talk, this is complicated further by the fact that regional monitoring stations don’t usually measure all of the pollutants elevated next to highways that we might be concerned about. Based on the emerging science, it is becoming clear that air pollutants next to highways pose health risks that may be larger than those of the regional ambient pollutants. This is concerning because even regions that are designated in regional PM attainment include very large numbers of near highway residents who are not protected. In fact, we know that 11% of US households are located within 100 meters of 4 lane highways. The more I study this area, the more it becomes clear to me that the most vulnerable populations are those least equipped to protect themselves. From the low income family living near a highway to the road construction worker pouring asphalt in the hot summer sun, socioeconomic status is clearly a marker for those most threatened by near roadway air pollutants.
- So…what are the motor vehicle pollutants of concern? Despite a huge list of hazardous air pollutants in the hundreds, scientists have been able to narrow it down to a handful of major culprits when it comes to near roadway pollutants. What you will see as we briefly go through these pollutants is that they are all by-products of the combustion of fossil fuels and because of their common source – vehicular emissions – they are typically highly intercorrelated.
- While there is a vast scientific literature that demonstrates both lung and heart effects from the inhalation of PM, the precise impact and mechanisms are still the subject of intense investigation. The image shown here provides an electron micrograph view of the structure of UFP taken from archival autopsy tissues from persons known to have died from the December 1952 London smog episode. The results of this paper highlighted the acute toxicologic importance of UFP in humans. Particles emitted from diesel engine are typically in the 0.1 micron range. These particles are small enough to pass through cell membranes and migrate into other organs, even the brain. So while there is a clear evidence for impacts on the lungs and heart, the emerging science is looking closely at impacts on the developing nervous system. One of the properties of UFP that make it difficult to study is the ability for other toxic air pollutants to adhere to the surface of these particles. This makes the so-called “speciation” of the particle very source specific and often difficult to characterize. For example, benzopyrenes, which we will talk about in a little bit, are a component of diesel particulate matter and are what impart the added risk of cancer. If you follow the evolution of the study of PM since the passage of the Clean Air Act you quickly see that, as we become more sophisticated and look at smaller and smaller particle sizes, that seems to be where the money is in terms of health impacts. In fact, all of the most recent studies suggest that the primary determinant of the biologic effects of UFP is their number or surface area, not weight. For example, one particle of PM10 has approximately the same mass as 1 million particles of 0.1 microns in diameter, but we know PM10 is much less hazardous, cleared by the body’s own defenses. Currently our regional monitoring of PM10 and PM2.5 is by mass. Concerns exist that this might not be the most appropriate method of evaluating and regulating the impacts of UFP. In fact, in some countries, there are proposals to shift the focus of regulation from particle mass to particle number.
- BC is a type of particulate matter that is an impure form of elemental carbon with graphite-like structure. It is the product of the incomplete combustion of fossil fuels, biofuels and biomass. A distinction that is worth clarifying about particulate matter here is that particles can be suspended in either a gas or liquid. When they are in a liquid that leads to pollution of our waters. When they are is a gas they are known as aerosols. Aerosols play a key role in both absorbing and scattering both solar and terrestrial radiation and can affect climate by changing the way radiation is transmitted through the atmosphere. In fact, BC is one of the most important light absorbing aerosol species and has been identified as the runner up to CO2 in emissions resulting in global climate change. In the developing world the burning of biomass for cooking and heating is a major source of BC emissions and for this reason has been identified as one of the low hanging fruit for mitigation strategies like replacing primitive cooking stoves with their more modern counterparts. As far as the health impacts of BC, direct effects are tied to the contribution they make to the burden of PM. Less direct but more global impacts on health will likely be seen as a result of their role in climate change.
- NOx are binary compounds containing nitrogen and oxygen. They are the result of combustion processes, particularly at high heat, of nitrogen-containing fuels such as coal and oil. NOx are highly reactive compounds. In particular, their reaction with VOCs in the presence of sunlight and heat, forms ozone, with well-known health impacts in the lungs, by inducing an inflammatory response. The principal site of toxicity of NOx is the lower respiratory tract. Recent studies indicate that low-level N02 exposure may cause increased bronchial reactivity in some asthmatics, decreased lung function in patients with chronic obstructive pulmonary disease, and an increased risk of respiratory infections, especially in young children. Extremely high-dose exposure (as in a building fire) to N02 may result in pulmonary edema and diffuse lung injury.
- CO is a colorless, tasteless, odorless, nonirritating, flammable and poisonous gas that forms as a result of the incomplete combustion of fuel. CO has been a major air pollutant of concern since the mid 1960s. The primary emission source, then and now, was from motor vehicles in urban areas. Inside homes, CO is emitted from natural gas fueled hot water heaters and home heating systems. In the 1960s, CO from these home heating sources was responsible for more poisoning deaths in the US than any other agent. The mitigation of CO pollution by the introduction of catalytic converters, which convert CO to CO2, has had huge impacts, with a 90% reduction in emissions since their widespread implementation. For our purposes, it is helpful understanding that acute clinical poisoning does not occur as a result of exposure to ambient concentrations of CO near highways. The annual mean CO concentration in the air in 1997 was below 9 ppm. However, in urban areas there are still high short-term peak CO concentrations that result in health effects Unlike UFP, CO has a well-known mechanism for its health impacts. It enters the bloodstream through the lungs and forms carboxyhemoglobin, which inhibits the blood’s capacity to carry oxygen, putting a strain on tissues with a high oxygen demand, such as the heart and the brain. The most noteworthy health effects from ambient exposures include reduced exercise tolerance, chest pain and EKG changes indicating poor oxygen delivery to the heart in patients with heart disease. For children, the concerns are greatest for the fetus and analogous to impacts seen on the fetus when mother smoke during preganancy, including a reduced birth weight and an increased risk of SIDS after birth.
- Polycyclic aromatic hydrocarbons have been pervasive in our galaxies for billions of years and play a significant role in star and planet formation. This artist's conception came from the NASA website and symbolically represents polycyclic aromatic hydrocarbons, seen in the early universe. NASA's Spitzer Space Telescope is the first telescope to see polycyclic aromatic hydrocarbons so early--10 billion years further back in time than seen previously. Spitzer detected these molecules in galaxies when our universe was one-fourth of its current age of about 14 billion years. PPAHs are formed by the incomplete combustion of organic materials, such as wood or fossil fuels. They can even be found in charcoal grilled hamburgers and burnt toast. These compounds are made up of three or more benzene rings and their relatively low vapor pressure results in their adsorption to PM in the atmosphere. The US DHHS classifies benzene as a human carcinogen and ong-term exposure to excessive levels of benzene in the air is known to cause leukemia. Benzo[a]pyrene (B[a]P) is the most studied because of its relatively high environmental levels and high level of toxicity. One study looking specifically at B[a]P found levels 2-fold higher both indoor and outdoor in high vs. low traffic areas.
- These various constituents can be measured in real time or near-real time using particle counters (UFP), analyzers that measure light absorption (BC and CO), chemiluminescence (NOx), weight (PM2.5 and PM10), and gas chromatography (PPAH and VOC).
- For the remainder of the time, I would like to tease out a few key points that came out of the article I gave you last time that I think are going to be key points to consider as we move forward in developing our monitoring plan. Several recent studies have shown sharp pollutant gradients exist near highways.
- There appear to be multiple factors that impact these air pollutant gradients.
- So while these pollutants are of significant health concern, we may not detect concentrations near roadway that are different form the ambient regional concentrations already there.
- Can we pick one of these as a surrogate for the others? And from the previous slide we also see that trying to detect differences in PM2.5 and benzene may not be useful.
- There are also many factors to consider when doing near roadway air monitoring. The authors of the paper highlights that many of these factors have rarely been taken into consideration in health outcome studies of near-highway exposure.
- And finally, I would like to bring us back to my comments at the beginning of the talk. For me, my involvement in this project is motivated by the disparate exposure that these children are going to have as a result of spending large amounts of time living and going to school near what appears to be a significant exposure risk. Much still needs to be learned about the complexities of these exposures and I would like to see our monitoring contribute some of the answers to these missing links. It definitely appears that pollutants next to highways pose health risks that may be larger than those of ambient pollutants. The strongest evidence for this comes from childhood studies of asthma and lung function, but more study needed to confirm existing findings. We also need studies that integrate exposures at school, home and during commuting times. Finally, we really need to look at those who live near highway, our least privileged and most vulnerable populations, and work hard to adequately address socioeconomic status as a potential confounder for the impacts we are seeing. Thank you.
- Gauderman et al published 2 noteworthy studies in 2005 and 2007 in the NEJM and Lancet, respectively, that followed a cohort of southern California children prospectively over 8 years.