I’ve been reading “The Uninhabitable Earth: Life After Warming,” an extensively researched account of where we are headed that begins with the warning: “It is much, much worse than you think.” When I got to the chapter depressingly entitled “Unbreathable Air,” I encountered the following shocking sentence: “Pollution has been linked with increased mental illness in children and the likelihood of dementia in adults.” Now I’ve seen all kinds of things associated with dementia: head trauma, assorted medications (anticholinergic drugs, anti-anxiety drugs, and anti-ulcer drugs), aluminum. Some of those links have become well-established over time, such as head trauma. Some of have been totally debunked, such as aluminum. Others are questionable and I’ve written about them on this blog (drugs). But air pollution? This was a new one to me.
Most likely, I figured, it would prove to be another spurious association. Probably, I thought, there was some other factor that was associated with both air pollution and dementia. The alleged connection would be like the link between washing machines and colon cancer—a favorite example of a “confounder” from my medical school epidemiology class. People who own washing machines, it turns out, do have a higher rate of colon cancer than people who don’t. But they also vary in where they live and what they eat, which is far more important than their possessing a washing machine. Surely air pollution was likewise a marker for something that did matter. But then, as I read on in Wallace’s book, I came to an even more dramatic statement: “An enormous study in Taiwan found that, for every single unit of additional air pollution, the relative risk of Alzheimer’s doubled.” This I had to look into.
The “enormous study in Taiwan” was published in a minor but respectable journal, the Journal of Alzheimer’s Disease in 2015. It was large: it was a cohort study of 95,690 adults aged 65 and older followed prospectively for 10 years beginning in 2001. Not only was it large, but it was a random sample drawn from Taiwan’s National Insurance Research Database comprised of 23 million people, or 99 percent of the entire Taiwanese population. Moreover, Taiwan has 70 EPA monitoring stations distributed over the island, allowing it to have reasonably accurate measures of both ozone exposure and small (less than 2.5 micrometers) particulate measure. Finally, the population is fairly stable over time, allowing for fairly good estimates of exposure based on home address. The conclusion? The risk of newly diagnosed Alzheimer’s disease (adjusting relevant co-morbidities such as stroke, hypertension, and diabetes) rose steadily with the rate of exposure to ozone or small particulate matter—going up, for example by 211 percent for each 10.91 ppb increase in ozone.
Taiwan isn’t the only place where a relationship between air pollution and dementia has been discovered. In 2017, a similar study entitled “Exposure to Ambient Air Pollution and the Incidence of Dementia: A Cohort Study,” appeared in Environmental International. Carried out in Ontario, Canada and involving a cohort of just over two million adults, this analysis attributed just over six percent of all dementia cases to air pollution.
Neither study is conclusive, but they’re awfully suggestive. I wondered if there had been any further work on this subject since Wallace wrote his book. Lo and behold, a systematic review was just published by Peters et al from Australia, also in the Journal of Alzheimer’s Disease. These authors found thirteen reasonably well-conducted studies bearing on the question. They concluded that small particulate matter (containing nitrogen) and carbon monoxide are both associated with an increased risk of dementia.
These reports are very disturbing in light of the Trump administration’s systematic assault on air pollution regulation. According to an article just published in the New York Times, 85 environmental rules are being rolled back, including 24 in the arena of air pollution. Of these 24, 10 have already been undone and another 14 are “in process.”
We already know that climate change will have an enormous impact on health, principally through its multitudinous indirect effects—for example, by causing drought, which in turn affects agricultural productivity, which in turn results in death. Now there may be another health risk to add to the list of adverse effects of environmental harm. Dementia is such an enormous public health problem that even measures that only slightly affect the risk of developing this devastating condition may be worthwhile. But the good news is that air pollution is an area where we can intervene. We even know how to. The last thing we should be doing is unraveling the progress we have made. So, speak up, tell your senators and representatives to act, and vote wisely to decrease the pollution that threatens us all.