7. Air Pollution Cuts Lifespan - How do we Know That?
Smoke and mirrors? Nah, just basic math.
The US Secret Service protects American presidents. But they can’t protect them from everything.
In 2015, former President Obama spent three days in New Delhi, India for high level meetings. The press breathlessly reported (pun intended) that Obama’s trip shed about six hours from of his life because of the profound levels of air pollution. How can we possibly predict this? Answer: lots of science, and some good math.
During this visit, particle concentration, an important measure of pollution, was fifteen times higher than the World Health Organization's air quality guideline. President Obama was surely surrounded by protective Secret Service agents to keep him safe at every moment of his trip. But breathing Delhi’s polluted air also surely put him at risk, even if he didn’t know it.
Scientists have long understood that air pollution - especially particulate matter and ozone - are especially toxic to humans. When we inhale these materials, they interact with our lung tissues, and some pollutants can enter our blood stream. Havoc can ensue which makes us sick.
But still, how do we get to six hours of life? This strangely precise answer comes from our understanding of how air pollution, which goes up and down in concentration, leads to people getting sick. For the most part, when pollution goes up, more people go to the hospital. Most recover, but for some, it is a fatal trip.
To do this, epidemiologists collect hospitalization records and air pollution measurements to identify periods that go up and down together. But it’s not a simple analysis, because hospitalizations often continue to persist well after the pollution occurred. And they begin to fade well after the pollution goes away.
There are some mathematical tools to help make sense of these relationships. These ‘lagged model’ studies, geek-speak for comparing hospital admissions with pollution concentrations from today, yesterday, the day before that, and so on, help us figure out how, and when, air pollution impacts human health.
When they’re done, they can describe a relationship that predicts how much increased hospitalization might be expected for different amounts of air pollution.
These relationships also vary based on age. An older and less healthy person, say an 85 year old, is usually more susceptible to pollution effects than a healthy 20 year old. There are certainly very healthy 85 year olds and very unhealthy 20 year olds. And don’t forget there are usually far more young people than older ones, so that has to be held to account as well. All of this has to be considered to figure out this messy relationship between pollution and health.
Some communities also have higher risks for different diseases just because their community was already at higher risk - and this happens independent of air pollution. For example, heart disease is much higher in the southern United States than elsewhere. So it’s likely that this population has both a higher baseline of heart disease hospital admissions, but may also be more sensitive to pollution.
All of this has to be considered by epidemiologists.
At the end of the day, we want a number that is simple to understand that answers a simple question: how likely is that that I will become sick or die from my exposure today.
Science Shows us How
A really important study in 2009 gives us some direction. This study found that across the United States, for every decrease of 10 microgram per cubic meter of particulate (the dust and soot floating in the air), lifespan increased by 0.61 years - an inverted relationship. Conversely, if you increase particulate matter by 10 micrograms per cubic meter (microgram/m3), one could assume this results in 0.61 years of life lost.
To a young-ish American president, this is about 1% of his remaining life, according to the Winton Programme for the Public Understanding of Risk based in the Statistical Laboratory in the University of Cambridge (UK). And 1% of a day is about 15 minutes. Again, this is per 10 microgram/m3, which means in a super polluted environment like Delhi, where particulate matter was more than 100 microgram/m3 (about 10x higher than Washington DC), you have to multiple the 15 minutes of life lost by 10 times; each day in Delhi therefore results in 2-2.5 hours of life lost. Multiple this by three, one for each day he was there, and we arrive at 6 hours. Okay, somewhere between 6-7.5 hours, but close enough.
The average Indian loses a little less than six years of their life, all because of lifetime air pollution exposure.
Despite the global headlines, there are two big issues with doing this that should make every epidemiologist shudder. First, it is not appropriate to predict an individual response from community data. In epidemiology, this mantra holds quite true: “your individual experience may vary”. The 0.61 year per 10 microgram/m3 is based on how millions of people across hundreds of counties in the United States responded to different amounts of pollution. Sure, on average, lifespan goes up with lower pollution. But that doesn’t mean the same thing applies for an individual who may have different behaviors and traits that make them more, or less, susceptible to pollution effects than the average community.
And second, one is assuming that air pollution exposure is causing people to become sick. Technically, this is known as ‘causality’ which is a nerdy description that some specific event leads absolutely to a specific effect - breathing polluted air causes you to die. Instead, most of our understanding uses data that describes an association, meaning that our understanding of why people get sick (and some die) is not absolute; other things could be causing people to become sick. The 2009 paper is one of association, not causality.
Don’t get me wrong: the weight of associative evidence for air pollution health effects is overwhelming. Air pollution exposure is strongly linked with sickness and death, even if we don’t have ‘causal’ evidence. Heck, it’s really hard to find causal evidence for gravity, but we all accept the association that when we drop a box on our foot, gravity was the most likely culprit. I don’t question that.
We worry about these sorts of things because we are constantly exposed to air pollution. It’s a persistent risk for everyone. But more for some than others.
It probably took you about 5 minutes to read this article. You may not have had time to read it, if you’d spent about an hour with the former President on that risky trip to India!
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