Should you fear more getting hit by a car, or breathing in its toxic emissions?
I recently engaged in a conversation with my colleague Julian Marshall about which kills more : Deaths by vehicles (cars plus trucks) from crashes or deaths by vehicles from air pollution. I of course defended the “honor” of transportation and said death by crashes was much larger, he is an environmental engineer looking at air quality, and thought that was very important too. (He also argues death from car emissions is just as “honorable” as actually getting hit by a car.)
It turns out, Deaths by Crashes is larger, according to official sources, but not as much as I expected. And, the answer depends on what metric you use.
|road injury + other transport injury||total air pollution|
|Years of life lost:||653||565|
Notes: Units: deaths per year per 100,000 people; DALYs [Disability Adjusted Life Year] per year per 100,000 people. Air Pollution is Ambient Particulate Matter air pollution and Ozone, not just transport related pollution deaths. Road injury plus other transport injury is comprehensive, the vast majority are road injuries, the large majority of which are due to car. This includes things like driving off a cliff as well as driving into pedestrians.
Those numbers include only fine particles (PM2.5) and ozone. Using the numbers above, air pollution is worse than crashes in terms of total deaths, but not in terms of DALYs and Years of life lost.
We also need to figure out the fraction of total air pollutio n that is due to road transport.
Just as estimating deaths from air pollution is inexact (no death certificate says “air pollution” as cause of death; instead rates of outcomes like heart attacks are elevated slightly in the presence of air pollution), so too it is inexact to estimate contributions from specific sectors (e.g., road transport) to the air pollution total. Some PM2.5 is emitted directly and some pollution forms in the atmosphere from chemical reactions of precursor emissions; the latter type (called “secondary PM2.5”) is harder to attribute.
An MIT study estimates that proportion at 25%. Notably they also come up with more annual deaths from air pollution at about 200,000 per year in the US (rather than the 100,000 or so the GBS study implies). Air pollution deaths (premature strokes, heart attacks, lung problems, and so on) on average shortens life by 10 years per person who dies from air pollution. Car crashes are more likely to shorten life of younger persons, hence the greater years of life lost per death.
The most useful number to look to, from an overall health standpoint, probably is the DALYs, since that number includes both mortality and morbidity (death and disease).Accounting for uncertainty, to a first approximation the numbers above are roughly comparable to each other, for the two causes.
I would conclude we should fear crashes more than air pollution from traffic, but we should not be sanguine* about emissions either.
*Our temperments of course are affected by the air we breathe.