Why Low Levels of Air Pollution Actually Cause the Most Harm

In this case, the idea is that air pollution hurts us little by little, but then the damage really starts to get bad at severe levels. This logic is implicit when people ask questions like, “How much saturated fat is safe?” Or “what’s a safe level of lead in my body?”

Or does pollution affect us in a more linear way?

That makes sense to me too. In that case, each step up in air pollution hurts us the same as the step before.
But scientists analyzed the effects of PM2.5 on death, and the data says both of my intuitions are wrong.

It’s the opposite! Lower numbers actually hurt more.

Or in the form of actual data, this is what it looks like. An increase in pollution at higher levels does LESS damage than increases at lower levels.

If that doesn’t make you poop your pants, perhaps you’re not such a stats nerd like me. But here’s why that graph is crazy.

Let’s say China has a bad year, and air pollution goes from 125 to 150 micrograms. Based on China’s population, that would mean 60,000 more people will die.

That’s a lot of people, probably more than have died on “The Walking Dead” total.

But that same increase—25 micrograms—would kill almost twice as many people (per capita) if it happened in China’s cleanest major city, Shenzhen.

How does that make any sense? Why would low levels matter more?

Here’s a metaphor: imagine getting hit by a baseball bat.

That first hit would hurt a lot—probably break some bones. But what about the fifth strike? The tenth strike? A lot of the damage has already been done, so the tenth strike doesn’t add much. (Sorry for the gruesome metaphor!)

Two Reasons Why This All Matters

Besides being a surprising finding, this datapoint actually has real-world implications.

1. The most important progress is still ahead
I lived through a year of Beijing smog when the average was over 100 micrograms. In 2018, Beijing averaged about 50 micrograms.

That’s amazing progress! Beijing cut its PM2.5 by 50 micrograms.

But because of the baseball bat principle, Beijing would save more lives if they improved from here. Oddly enough, improvement becomes more valuable once things are pretty clean.

2. If you’re in a “safe” city like I am, you should be worried.
For my day-job, I live in Chicago. When I see graphs like this, I feel pretty good about myself. I feel safe.

But the new studies on health effects at low levels of pollution have changed the way I think about safe air. The baseball bat principle could explain why recent studies have been finding meaningful health effects on blood pressure and diabetes.

Data has even found meaningful effects comparing 12 micrograms to 7 micrograms. Less is better.