I read an interesting comment in a NY Times article today, the gist of which is that breathing the recent smoky air, at its peak, was like smoking six cigarettes a day.
And I thought to myself a) sounds like somebody made that up, and b) is that even remotely close to being true?
Short answer: No, not even close. At least, not if you’re just tallying up the total weight of particulates inhaled. See the last section.
Part of the ambiguity in this question is whether you’re simply talking about the total weight of particulate matter inhaled — a fairly direct calculation — or whether you are trying to infer the net impact on health, from those particles — a far less clear calculation.
Virtually every estimate you will read, comparing air pollution to cigarette smoking, is an indirect estimate that tries to compare them based on presumed effect on health. Typically, those are based on the observed relationship between cigarette smoking and lifespan, compared to the observed relationship between air pollution and lifespan. Essentially, those are based on comparisons of epidemiological studies.
Just to be clear, those indirect estimates based on health effects appear to show a vastly higher equivalence between air pollution and cigarette smoking. That is, they make the claim that routine levels of air pollution are the equivalent of smoking several cigarettes a day. That’s much higher than you would get, simply calculating the weight of particles inhaled from air pollution, versus from smoking.
I address that at the end of this post.
For now, I just want estimates of the total weight of particles. What weight would have been breathed in, in 24 hours of New York City’s worst air. And how much do you inhale, if you smoke a cigarette?
If you do the math, you’ll find that spending 24 hours in the worst of NY City’s recent bad air meant inhaling the same quantity of particulates you’d get from smoking about one cigarette.
By contrast, commonly-used air-pollution-to-cigarette equivalences suggest a vastly different far more eye-popping equivalency. New York’s peak PM 2.5 reading of 460, if maintained for 24 hours, would be roughly the equivalent of smoking a pack of cigarettes, using the commonly-cited Berkeley Earth estimate that 22 micrograms/cubic meter PM 2.5 is the equivalent of one cigarette per day (reference).
Suffice it to say that, as a health economist, I find the method they used to derive that to be subject to some uncertainty. Just Google air pollution deaths, and you’ll see that even the most basic estimates are all over the map.
So I’m sticking to something far more basic. What’s the weight of inhaled particles, compared to what you’d get from a single cigarette.
Details follow.
The calculation
Determining the total weight of air pollution particles inhaled is straightforward. Take the concentration per cubic meter, and multiply by the average number of cubic meters a person breathes in a day.
(This of course ignores the fact that you exhale some of those particles. So this isn’t the weight that you retain in your body. It’s just the amount that you inhaled.)
The clear-enough answer here is that 24 hours of breathing the air, at the peak of New York City’s bad air, would have resulted in inhaling about 11 milligrams of total particulate matter.
That’s an estimate for particles of all sizes. If you focused narrowly on PM 2.5, it would be about 5 milligrams.
Now for the hard part. How much particulate matter do you inhale when you smoke a cigarette?
You can get a hint that my answer is going to differ quite a bit from the “health impacts” answers just by looking up the weight of a cigarette. For reference, a typical cigarette weighs a gram, or 1000 milligrams, per the WHO. Of that, about 15 percent if water (reference).
So the dry organic matter in just one cigarette weighs about 70 times as much as all the particulates of all sizes that would have been inhaled if you lived one entire day at the peak of New York City’s recent air pollution.
The only question is, what part of that weight ends up being inhaled as cigarette smoke?
Estimate 1: 10 to 40 milligrams (cited in this reference).
In short, you would inhale as much particular matter from smoking one cigarette, as you would living for 24 hours at the peak of New York City’s recent smoky air.
To benchmark, an estimate based solely on inhaled weight appears to equate PM 2.5 level of 85 with about 0.3 cigarettes a day (cited by Berkeley Earth). That would make New York’s peak of 460 equivalent to about 1.6 cigarettes per day.
So, either by my direct calculation, or by reference to a different calculation, the weight of particulates inhaled at the peak of the recent bad air would be in the neighborhood of the dose you’d get from smoking … roughly one cigarette.
Why the big discrepancy?
I’m pretty sure my calculation of the weights of materials is roughly right. And I’m still pondering the huge difference between the air-pollution-to-cigarette equivalency based on weight of particulates inhaled, and that based on apparent health effects derived from epidemiological studies.
Again, let me emphasize that the commonly-cited Berkeley Earth estimate (based on apparent impact on health) is an order-of-magnitude higher than the simple estimate based on weight of materials. That shows up here (where the Berkeley-style estimate would be a pack (20 cigarettes) a day, versus my weight-based estimate of one cigarette a day. And it shows up in Berkeley Earth’s estimate of the impact of air pollution in Beijing, where their estimate was again about an order-of-magnitude higher than an estimate simply based on weigh of particulates inhaled.
Here’s the weird bottom line. If we assume that both results are right — both the estimate by weight, and the estimate by apparent impact on health — then this probably implies one of two things.
First, if there’s a linear dose-response relationship — if twice as much smoke is twice as bad for you — then this discrepancy would imply that air pollution particulates are vastly more toxic than cigarette smoke. Roughly speaking, if both estimates are right, a milligram of particulates from air pollution has the same toxicity as 10 milligrams of cigarette smoke.
That does not seem quite plausible, to me. Possible, sure. But my understanding is that cigarette smoke is some pretty toxic stuff. I find it hard to think that (e.g.) burning coal in a power plant (plus smokestack scrubber), or wood in a forest fire, could be that much materially worse than cigarette smoke, where almost all the particulates are the fine PM 2.5 particulates.
Alternatively, we could explain the same results by a declining dose-response relationship. Cigarette smoking, in the U.S., consists of a small number of adults who get a large daily dose of smoke. Air pollution particulates, by contrast, consists of everyone getting a small daily dose of smoke. If the bulk of the health impacts come from that first little bit of smoke … then sure, you could get small amounts of air pollution equaling the health impacts of large amounts of cigarettes.
And, data from cigarette smokers supports that. Here’s a study of persons who were non-smokers, occasional (non-daily) smokers, and daily smokers.
The occasional smokers consumed just 8% as much tobacco as the daily smokers. But with that modest rate of consumption, they incurred almost half as much excess mortality risk. For smokers, it absolutely is true that most of the health impacts come from just a little bit of smoking.
Stated another way, smoking a pack a day (600 a month) is only about twice as bad for you as smoking about 2 cigarettes a day.
And that, I think, it was explains the big discrepancy between air pollution estimates based on weight of particulates inhaled, and estimates based on presumed health effects of smoking versus air pollution. As-consumed, it probably is true that the typical milligram of cigarette smoke has much less additional health impact than the additional milligram of PM 2.5 in the air. But that’s only because once you’ve had your first couple of cigarettes of the day, the damage is done, and the remaining pack or two hardly matters.
(And so, weirdly, if you’d filled in the blank above with 2 cigarettes a day, or you filled it in with 20 cigarettes a day, really, there isn’t much difference between those two answers. In real, actual, cigarette-smoking terms. That’s per the table just above. What you can’t really do, strictly speaking, is to do that equivalency calculation based on simple averages, and expect the results to be meaningful as an expression of actual cigarette smoking. That’s because the cigarettes-versus-health curve is so strongly non-linear.)
The more I look around on this issue, the more I see that scientists seem to be coming to more-or-less the same conclusion. The problem with airborne particulates isn’t the occasional peak days, as we just had. It’s that even low, chronic levels of PM 2.5 in the air can lead to significant negative impact on health, when inhaled over a lifetime.
