by Paul Ehrlich (Clean Air Issues Coordinator; phehrlich-nj@outlook.com)

The generation of air pollution by internal combustion engines is a major factor in air quality. Much less notice has been given to the concentrations of pollutants inside vehicles, which depends on pollutants generated by the vehicle itself ("self-pollution") plus other nearby vehicles as well as regional/local ambient air quality. It turns out that all these issues have been intensively studied and the results, on the average, are not good. I’ve written before (New Jersey Sierran, Oct-Dec, 2017) about air pollution effects on cyclists who share the road. In this article effects on children on school buses and car drivers as well as cyclists is examined in more detail.

School Buses

Most worrying are the results of school buses with diesel engines: pollution in the cabin is higher than in the air ahead of the bus. Notably, the effects of pollution on children are worse than adults. The exact levels of pollutants are quite variable because of a complex interplay of many factors including age of the bus, condition, type of engine, number of open windows, weather, and type of route (urban or rural, traffic volume, hills, etc) in addition to the level of pollutants from sources outside the bus (both ambient and caused by other vehicles). Some results appear counterintuitive, such as an increase in cabin pollutants when the bus is idling. Adding to this unhealthy commute is the time spent waiting to step onto a bus that is idling, an especially toxic time when there is a group of buses.

Gas-Powered Cars

Gasoline-powered cars also produce high interior concentrations of dangerous pollutants. As with buses, results can be quite variable depending on the car model, age, percentage of air recirculated, etc. In one study self-pollution is the source of 30% of PM2.5, particulate matter pollutants that include the especially toxic subset called ultrafine particles (also known as nanoparticles). In addition, New Jersey drivers frequently experience traffic congestion . A study of pollution at traffic intersections showed that stopping at a traffic light with many other vehicles can result in as much as 29-times higher pollutant levels than when traffic flows freely. Thus, spending 2% of a car ride at traffic lights could result in 25% of the total pollutant exposure experienced on the outside of a car. How much gets inside is, of course, variable, but likely to be much increased.

Bicyclists

Even though the air inside cars is frequently more polluted than the ambient level on a roadway can we conclude that bicycle riders are better off? Apparently not according to one study: cyclists inhaled more pollutants than car drivers (higher pollutant levels inside cars was confirmed in this study). A major contributor to this result is the much higher volume of air that enters cyclists' lungs. Predicting the effect on cyclists is difficult. Air pollution in urban areas is known to be extremely variable in time and place: even 25 yards can separate areas with significantly different pollution levels and, as noted above, a traffic light can make quite a big difference. Portable pollution monitors were used to study the exposure of cyclists in Piacenza, Italy (a mid-sized city) according to the type of lane on which they were riding: same lane as cars, separated lane for cyclists, and a "green path lane" which was about 45 yards wide with no cars. The average concentrations of nanoparticles in the green path lane was about 43% of the concentrations in the unseparated lane while concentrations in the separated lane were improved less than in the green path. However, there was no difference in the various lanes for the traditional measures of particulate pollution (PM10, PM2.5, PM1), which is likely due to their lower correlation with traffic volume and their high background concentrations in this area of Italy.

Determining the healthiest route

Studies addressing the health effects of "active commuting" (commutes by walking or cycling) frequently compare the benefits of exercise to the downside of exposure to pollution. Results are dependent on many factors due to the great complexity of pollutant exposure. Besides distance from traffic and time of day (frequently associated with time of traffic congestion), other determinants of health risks from pollutants include temperature, wind velocity and direction, seasonal effects, structures on the sides of roads (such as urban "canyons" that affect pollutant dispersion), nearby trees and other vegetation, and characteristics of vehicles (diesel-, gasoline-, or electric-powered plus additional vehicle properties). A characteristic that is indicative of the difficulty in predicting pollutant levels is the amount of air trapped in the wake of a vehicle: pollutants in that space can react to form new pollutant molecules that could have different effects. Thus, determining the healthiest route is best estimated by measuring levels of several important pollutants multiple times.

Summary

Riding in cars and cycling on roads have adverse health effects of varying magnitude. For cyclists air pollution could overcome the benefits of exercise. The best solution in the short term is to avoid congested areas and to travel at times when streets are less crowded. Electric vehicles, especially mass transit vehicles, will help significantly. In the long term adverse health effects will be significantly reduced by improving mass transit options, increasing the percentage of passengers transported by electric-powered vehicles, having cycling-only lanes lined with trees and shrubs, and creating electrification zones (areas where only electric vehicles are allowed).