|Above: Exhaust flows out of the tailpipe of a vehicle at 'Mufflers 4 Less', July 11, 2007 in Miami, Florida. Photo by Joe Raedle/Getty Images.|
Death certificates never list air pollution as the cause of death. Nevertheless, air pollution is a huge and silent killer: about 3 million premature deaths per year globally are due to outdoor air pollution. Between 91,000 and 100,000 air pollution deaths per year occur in the U.S., according to separate studies done in 2016 by the World Bank and the Health Effects Institute (a U.S. non-profit corporation funded by the EPA and the auto industry.) This death rate is nearly triple that of car crashes, which claim about 35,000 U.S. lives each year.
Air pollution deaths are calculated using epidemiological studies, which correlate death rates with air pollution levels. Air pollution has been proven to increase the incidence of death due to stroke, heart attack and lung disease. Since these causes of death are also due to other factors—such as life style and family history—we typically refer to air pollution deaths as premature deaths. A premature air pollution-related death typically occurs about twelve years earlier than it otherwise might have, according to Caiazzo et al., 2013. The two pollutants known to be deadly for large numbers of people are ground-level ozone and particulate matter—especially the tiny particles known as PM2.5 (those that are less than 2.5 microns or 0.0001 inch in diameter). PM2.5 pollution is thought to kill about ten times more people than ozone.
Vehicles are responsible for about 1/4 of U.S. air pollution deaths
Automobiles in the U.S. cause about 26% of all air pollution deaths in the U.S., according to a 2013 MIT study. This translates to roughly 25,000 vehicle-related air pollution deaths each year, if we assume 100,000 total U.S. air pollution deaths per year. PM2.5 is the primary cause of traffic-related health impacts, but traffic-related PM2.5 exposure varies greatly depending upon city. Motor-vehicle PM2.5 contributions are estimated to be 49% in Phoenix, AZ and 55% in Los Angeles, CA, but just 5% in Pittsburgh, PA, where very high background levels of PM2.5 exist due to power plant emissions and long-range transport. Outside the United States, estimates of the motor-vehicle contribution to PM2.5 range from 6% in Beijing, China, to 53% in Barcelona, Spain.
The greatest risk of a vehicle-related air pollution death is for people living close to an expressway or major road. The danger zone may extend as far as 500 meters downwind of a major expressway, but only 100 meters or so upwind. The greatest PM2.5 levels are found less than 50 meters from a busy road; one study found a 50% decrease in PM2.5 within 100 to 150 meters from a road, and a decay to background concentrations within as little as 50 m has been reported. These numbers mean that a large number of people living in urban areas are subjected to air pollution levels much higher than the official EPA pollution monitors report. In 2000, 44% of the population of Los Angeles and 45% of Toronto lived within 500 meters of an expressway or 100 meters of a major road.
|Figure 1. A week’s worth of PM 2.5 measurements taken in August 2003 at two transects along Highway 401 in Toronto, which consistently ranks as one of the top three busiest highways in North America (annual average daily traffic flow was in the range of 365,000 to 395,000 vehicles per day at the two transects). The data shows that PM 2.5 levels were more than double the background values within about 50 meters of the expressway, and were elevated up to 200 meters downwind of the road. Upwind PM2.5 was very close to background levels. Image adapted from: Beckerman B, Jerrett M, Brook JR, Verma DK, Arain MA, Finkelstein MM. 2008, Correlation of nitrogen dioxide with other traffic pollutants near a major expressway, Atmos Environ 42:275–290.|
High exposure to pollution while driving
Levels of PM2.5 pollution along busy roads are roughly double ambient levels, so you are greatly increasing your air pollution exposure if you drive with the windows open. The highest PM concentrations occur in congested traffic or when driving behind a heavy diesel-driven vehicle. Your exposure is not reduced much by driving with the windows up, if the car’s ventilation system is operated to bring in outside air. While cars are equipped with a cabin air filter which reduces levels of harmful particulate matter sucked in from the road, these filters only reduced PM 2.5 levels inside the car by 29% compared to outside pollution levels, according to one study that looked at a wide range of commonly driven 2010 - 2013 model year cars. This is not much better than when the filter was removed, which resulted in a 22% reduction. Cabin air filters are only required to be effective for particles of 0.3 microns or larger in diameter, and significant lung damage is done by ultra-fine particles smaller than that, which are emitted by cars in great numbers.
To greatly reduce your air pollution exposure while driving, operate the ventilation system under recirculation (RC) mode (you may need to run the air conditioner to keep the windows from fogging up.) Recirculation mode can achieve a reduction of about 90% in PM2.5 levels, and it takes just three minutes once RC mode is turned on for the air to clear to levels typically breathed in an office. However, recirculation mode also causes passenger-exhaled CO2 to accumulate rapidly above 2500 ppm in the car--over six times the ambient air concentration. Exposure to high CO2 concentrations of 2500 ppm for several hours can significantly reduce decision-making performance, and may cause drowsiness. If you are on a long drive in heavy traffic, it is probably a good idea to turn RC mode off for a few minutes every half hour or so if you are concerned about staying alert and making good driving decisions. Some new vehicles have the feature of periodically switching off the recirculation and bringing in air from the outside for a short period of time during the recirculation setting.
Children on diesel school buses breathe unhealthy air
A landmark 2002 study by the California EPA found that school children commonly breathe unhealthy air from diesel school buses used to transport them to school. The extra diesel exhaust breathed by a student over a 13-year period of being bused to school increased their lifetime cancer risk by 4%, and increased the risk of being hospitalized for asthma by 1%. The extent of a bus’s own contribution to unhealthy air inside the bus appeared to be highest when windows were closed for the older diesel buses; the worst air quality was in the back seats of the bus.
Vehicle pollution may contribute to dementia
In 2017, a study of residents of Ontario, Canada, found that those who lived within 300 meters of a road with heavy traffic had a higher risk of dementia, including Alzheimer’s disease--though not Parkinson’s disease or multiple sclerosis. The strongest association was among those who lived in major cities within 50 meters of a busy road, and who had never moved. The study’s results meant that 7–11% of dementia cases in patients who live near major roads may be attributable to traffic. Nitrogen dioxide and PM2.5 were the pollutants connected to the increased dementia cases near roads, but did not fully explain this increase, leading the researchers to suspect that traffic noise or other pollutants may also contribute.
New ways to track air quality through WU
As detailed in Bob Henson's April post, It’s Time to Give Air Quality the Attention It Deserves, tracking the ups and downs of weather in your own neighborhood—through your own personal weather station or your neighbor’s PWS—has been a core part of WU since our earliest years. We’ve now embarked on a concerted effort to examine the growing options for consumer-level AQ measurement. One of our first goals is to find out which platforms offer the best mix of affordability and quality. As this process evolves, we’ll be sharing what we find so that our community can help build a new network of personal air quality stations.
We are also adding key air quality indexes to many of our “dashboard” pages, which summarize the data gathered at each personal weather station in our network. For example, we’ve recently added AQ data for around 1500 stations across North America that are part of the U.S. Environmental Protection Agency’s AirNow network. For each of these sites, the weather history graph now includes hourly levels of various criteria pollutants, depending on location (see example below). AirNow stations are located in both large cities and more rural areas, including Minneapolis, MN; Provo, UT; and Washington, DC. AirNow stations vary in the types of meteorological and AQ data monitored.
Figure 2. Hourly measurements of PM10 (top, in micrograms per cubic meter) and ozone (bottom, in parts per billion) at station KCASACRA81 in downtown Sacramento, CA, on Friday, March 17, 2017.
This week is Air Quality Awareness Week, and the EPA has a great website dedicated to it.