Key Points

• Deadly smog events in the 20th century led to important insights and research into
   the toxicity of outdoor air pollutants.

• Early studies of air pollution, such as the Harvard Six Cities Study, used data from a
   single air monitoring station to assign a pollution exposure value for all people
   living in a city, whereas newer studies used data from more diverse sources
   to assign specific exposure values for people living in different parts of a city.

• New ways to estimate air pollution, such as land use regression, have enhanced
   estimates of ambient air pollution concentrations to within a few city blocks.

• Traffic-related air pollutants vary in kind and quantity over time and from one
   neighbourhood to another, but usually include nitrogen dioxide (NO₂), sulfur dioxide
   (SO₂), and particulate matter (PM).

• Particulate matter less than 2.5 microns in diameter (PM_2.5) is small enough to
   reach the lungs and is strongly associated with adverse health effects.

• Exposure to all traffic-related pollutants is most commonly, although not exclusively,
   associated with respiratory disease and cardiovascular disease.

• Paying more attention to socioeconomic status has led to advancements in our
   understanding of the impact of air pollution on health.