Non-linear Effects from Wildfire Smoke

There are falling marginal costs from pollution

Air pollution is bad for our health. As anyone who’s tried to breathe on those hazy summer days when the smoke drifts down from Canada and the sun glows orange will tell you, it sucks. Air pollution is an especially important problem in the developing world — poor air quality in Delhi likely kills 12 thousand people every year. It is one of the ways in which climate change will impact humans. By making wildfires more likely, even non-coastal regions will be adversely affected.

What is uncertain is the curve relating particulate exposure and health harm. It is possible that the two are linearly related, but it is not implausible that there might be not much difference between a low level of pollution, and absolutely none at all. Our present regulatory standards are based on the assumption that the curve is somewhat convex — below a threshold, it is not worthwhile reducing pollution further. Note that if the danger from pollution were perfectly linear, this would imply that action on pollution is equally needed at all levels of pollution, and where regulation occurs is ultimately determined by where pollution is reducible at least cost, not where health benefit is greatest.

A new paper, “The Nonlinear Effects of Air Pollution on Health: Evidence from Wildfire Smoke”, by Miller, Molitor, and Zou, uses wildfires to better estimate the shape of particulate emissions’ effect on health. They use the smoke plumes from wildfires as an instrumental variable. Wildfires are the ideal instrument for this, because whether or not you are currently underneath a smoke plume is plausibly unrelated to whether or not you were a week ago or yesterday. One could imagine that if smoke pollution rose during a season, it might be confounded with things like flu season. Sudden shocks are the ideal way to determine immediate impacts.

Some key facts. First, wildfire plumes did indeed sharply increase the level of particulate matter in the air. Being directly underneath the smoke plume increased exposure by 50-150%. These smoke plumes are not a small source of particulate matter either, accounting for 18% of the total particulate matter in the air in the US.

Second, exposure to the smoke causes serious adverse health events. One day of smoke exposure causes .51 additional deaths and 9.7 ER visits per million adults. This is 1 out of every 240 deaths, and 1 out of every 145 ER visits. This implies a population wide impact of 10,070 premature deaths, and 191,541 ER visits every year from wildfire smoke. These are not due to simply hastening the deaths of the very weakest by a few weeks — the deaths from wildfire smoke plumes were not compensated by lessened mortality in the weeks after.

Lastly, and perhaps most importantly, the shape of the effects from particulate matter was concave. Health risks saw the largest increase when changing from small to medium shocks, but then leveled off as the shocks got really big. This means that the marginal cost of additional pollution is actually decreasing. This may imply really big changes in how we should optimally treat pollution. Eliminating small shocks entirely may be much more valuable than reducing big shocks to moderate shocks. Aggressive firefighting, which aims to prevent even small blazes, has gone out of style, as it simply makes the fires which do happen much bigger. It is possible that, once you take the health consequences of air pollution into account, it is better to try and extinguish all fires, and live with the few big ones that escape contain. It also means that our regulatory standards, which focus on mitigating to below a threshold, and do not care below that, are misguided. It continues to be bad, even at small doses.

Some words of caution, however. This may be due to adaption. Once it crosses some threshold, it becomes worthwhile paying attention to, and people take corrective action like staying home, buying an air purifier, and so on. Smaller events see people take no action at all. If this is the case, then we are not seeing the idealized shape of particulate matter’s effect on health. It is still the policy relevant relationship, though. We should also do more to educate people about the dangers of air pollution. Even small amounts are still harmful, and you oughtn’t ignore it unless it blots out the sun. This goes for you, too, dear reader. Take contamination more seriously! Small investments can have large returns.