Friday, September 11, 2020

Indirect mortality from recent wildfires in CA

[This post is joint with Sam Heft-Neal]

Wildfire activity in the US West coast has been unprecedented in the last month, with fires burning larger and faster than ever experienced.  Multiple communities in the paths of these fires have been entirely destroyed, and wildfire smoke has blanketed huge swaths of the West Coast for weeks.

While media coverage on fire impacts has justifiably focused on the lives that have immediately been lost to the fire, the total costs in terms of human lives and health is likely far larger, due to the immense amount of smoke that has been inhaled over the last 3 weeks by the very large number of people living on the West Coast.  

How large might these effects be?  We know a lot about how exposure to certain air pollutants -- in particular, PM2.5 -- affects a range of health outcomes, including mortality.  Recent wildfire activity has led to a massive increase in PM2.5 above normal levels.  As anyone who lives in CA or has watched the news knows, air quality has been terrible, and the monitoring data of course bear this out. Below is a plot of the deviation of daily PM2.5 in 2020 from the previous 5yr average, beginning Aug 1 and going through Sep 10th 2020.  This is based on station data from EPA AirNow, and we average over each reporting station in each location on a given day. 

Difference in PM2.5 on each day, 2020 relative to 2015-19 average. Data are from EPA AirNow, and represent averages over all stations reporting in each area.

For most locations in CA, PM2.5 was 10-50ug/m3 higher than normal on many days, which is a massive increase above average (which in the Bay Area is typically ~ 10ug).  For simplicity, we are going to attribute all the excess PM2.5 in Aug/Sept 2020 (relative to 2015-19 average) to wildfires. While this is probably not completely correct, it's not outlandish: the huge observed spikes were definitely due to the wildfires.  Before about Aug 15, PM2.5 was running somewhat below recent averages across much of CA; fires kicked up in mid august, and PM2.5 levels shot through the roof. 

What are the health consequences of this change in exposure? The best paper on the short run mortality and morbidity consequences of PM exposure is a recent (very impressive) paper by Deryugina et al published in the American Economic Review.  Using very detailed Medicare data, they estimate that a +1ug PM2.5 increase on one day increases deaths over the next three days by 0.7 per 1 million people over 65+, and increases ER visits among the elderly by 2.7 per million people.  We emphasize that these result is for all PM2.5, not just PM2.5 from smoke; the literature is still undecided whether the latter has differential health effects, so we will just assume here that the effects are similar. 

For simplicity, let's say total PM2.5 values (excess PM2.5 since Aug 1, summed over days) were 300ug/m3 above normal across CA due to the wildfires. This will be a little too high for LA, but will be way too low for most of NorCal and the central valley. 

There are about 6 million people aged 65+ in CA. Applying the Deryugina et al estimates to the change in PM and the exposed population, we arrive at 1200 excess deaths (deaths that would not have happened otherwise) and 4800 additional ER visits among the elderly.  If we use the Deryugina et al estimates of how much one day of additional PM2.5 increases mortality over the next month (not just next three days), the estimated number of deaths rises to 3000.  This is just in CA alone!  And just for people aged 65+.  Oregon and Washington are being hit very hard right now too, and non-elderly are also surely affected.  So this is likely a substantial lower bound on total health costs. 

There are a number of caveats to this calculation that are important to consider. In most cases they suggest these numbers above could be conservative. Note that we will never know the "true" number because we don't observe the counterfactual -- i.e what would have happened in the absence of the dramatic decline in air quality.  But the purpose of the Deryugina et al paper is to get us as close as we could possibly get to that counterfactual.  Some caveats:

  • Again, we attribute all the change in PM2.5 in 2020 (relative to 5-yr avg) as due to wildfire.  While this is probably not fully correct, the big spikes were definitely from wildfires.  Further analysis will help clarify this. 
  • PM2.5 from wildfire smoke might not have the same effect as overall PM2.5 (only some of which comes from wildfire).  The science isn't clear on this yet.
  • We are in the midst of the COVID pandemic, and this has as-yet-unknown implications for our understanding of how air pollution affects health outcomes. Early evidence seems to suggest that poor air quality could worsen COVID-related outcomes, and if that's the case then our numbers above could be lower bounds.
  • Numbers above only look at elderly mortality and ER visits, and it's likely that other groups are very substantially affected (e.g. the very young, those with pre-existing cardiovascular or respiratory conditions).   Again, above numbers likely a dramatic lower bound on overall health consequences for that reason.
  • We don't know a ton about health responses at super high exposures.  Does it tail off?  Does it get way worse?  You can find papers telling you both.  Right now we're assuming response is linear, which is consistent with a lot of the literature. 
  • We also don't have a full picture of "displacement" - i.e. if someone passes away due to air pollution exposure, maybe they were already extremely sick and so would have passed away in the near term anyway.  Deryugina et al explore this to some degree by looking at 3-day (and longer) responses to a single day of pollution increase.  They find effects of 1-day exposure over the next month to be about twice as big as the 3-day estimates we use, so again this could suggest our numbers are a lower bound. 
  • Effects could also amplify over time, as its probably the case that exposure to 7 days in a row of terrible air is worse that exposure to 7 days spread out.  Our numbers do not consider this, nor do we know of good estimates in the literature that would help with this. Please point them out if so!

These overall effects can be in large part attributable to climate change, which has dramatically increased the likelihood and severity of wildfire.  Understanding this pathway of climate impacts is, we think, underappreciated, and something that we're working on (along with lots of others!).

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