Air pollution is a killer. It is responsible for more deaths than better known risk factors, such as alcohol use, physical inactivity, or unsafe sex.

Risk factors don’t usually kill you directly. Almost nobody steps off an airplane in Delhi or Beijing and dies from inhaling a breath of polluted air. Instead, risk factors make it more likely that you will get a disease, and that disease will either kill you or disable you. Using this logic, it is possible to say that all deaths are “caused” by some combination of risk factors, which lead to the specific diseases or events that kill the individual.

Figure 1. Number of deaths worldwide attributable to 17 risk factors. Source: Institute for Health Metrics and Evaluation, 2018.

Public health officials estimate the number of deaths that result from (are caused by) the various risk factors. For instance, if a person has high blood pressure and high blood glucose, and that person dies at age 68 instead of age 78, which was the person’s life expectancy, then that person lost 10 years of expected life. Public health officials try to figure out how many of those 10 lost years were attributable to the high blood pressure, and how many to the high blood glucose. They then assemble that data into a statistic that represents how many deaths per year were caused by each.

Figure 1 shows the number of global deaths per 100,000 in population that are attributable to the most important risk factors. Air pollution is 4th, behind high blood pressure, dietary risk (unhealthy food), and tobacco use. The total number of deaths attributed to air pollution in 2016 was 6.1 million, or 9.6% of all deaths from all risk factors.

The primary diseases to which air pollution contributed were heart disease, stroke, chronic lung disease (including asthma), and respiratory infections. Air pollution was responsible for more deaths than many better known risk factors such as high blood glucose, high cholesterol, alcohol and drug use, malnutrition, and unsafe sex (HIV/AIDS, etc.) In fact, despite all the publicity that unsafe sex gets, only 1.2 million deaths were attributed to it worldwide in 2016. Don’t get me wrong, 1.2 million deaths are a terrible thing, but air pollution kills more than 5 times as many.

Figure 2. Disability-adjusted life years (DALYs) attributable worldwide to 17 risk factors, 2016. Source: Institute for Health Metrics and Evaluation.

Risk factors don’t have to kill you, they can also cause disability. A person with a disability may live for many years before dying, trying to cope with that disability every day of every year. Thus, in public health terms, a disability incurred early in life has somewhat different implications than a disability incurred late in life. The Global Burden of Disease estimates not only the number of people with a disability, but multiplies it by the length of time they will have to live with it. This estimate is called the disability-adjusted life years (DALY). Air pollution is the 5th most important risk factor for DALYs, with malnutrition having vaulted into the lead position. (Figure 2)

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Figure 3. Deaths per 100,000 attributable to air pollution, by country, 2016. Source: Institute for Health Metrics and Evaluation.

Figure 3 shows a map of the world onto which the number of deaths per 100,000 from air pollution has been charted. North Korea loses more of its population to air pollution than any other nation, followed by the Central African Republic, Georgia (the country, not the state), and Afghanistan. This may surprise many readers, as we often think of air pollution being a function of industrial emissions in large cities, but in many developing nations, this is not the case. Readers of this blog know that particulate matter is the most dangerous of the 6 criterion pollutants. In developing countries, the people often use fires inside the home for cooking and warmth. The fires are smokey, and the homes are poorly ventilated, resulting in high levels of particulate air pollution. In addition, blowing fine mineral particles play an important role in some desert countries.

The United States has a death burden from air pollution of 32.6 per 100,000: low, but not one of the lowest in the world.

The above data looks at number of premature deaths caused by air pollution. Another way to look at the data is by asking how much air pollution shortens an average person’s life. Just such a study recently appeared (Apte, et al., 2018). Supplementary data associated with that article estimated the average life span in the United States to be 78.8 years, and PM2.5 will take about 4-1/2 months off of the average life expectancy. That was 22nd best in the world. Sweden had the lowest loss of life expectancy from PM2.5, about 1/3 that of the USA, while Bangladesh had the highest, almost 5 times that of the USA.

So, what diseases has air pollution been implicated in? We know from the above that it is known to cause disability and contribute to early death. We know that it contributes to the development of heart attack, stroke, chronic lung disease (including asthma), and respiratory infection. These relationships have been well documented, and are strong. But air pollution has also been implicated in diseases you wouldn’t expect. It has been implicated in a host of neuropsychological conditions, from increased signs of inflammation in the brain, to increased rates of Parkinson’s disease, to reduced IQ, to increased risk of ADHD, to increased rates of autism spectrum disorders, to reduced motor functioning. It has been implicated in hastening cognitive decline late in life. It has been implicated in the development of obesity and type 2 diabetes.

My impression from the studies of air pollution’s relationship to mental functioning, obesity, and diabetes is that their conclusions should not be heavily relied upon, as confounding variables undercut the comparisons the authors try to make. Even when their findings hold up, air pollution seems to play only a small role in most of these diseases. Many of the studies enrolled large numbers of subjects, making it possible to find statistically significant results with small differences of questionable importance. This is sometimes hidden from view by reliance on the relative risk statistic. Relative risk compares the risk in one condition with the risk in another. For instance, suppose 2 people out a million of develop a disease. If people are exposed to air pollution, however, then 3 people out of a million develop the disease. The relative risk is 3/2 = 1.5, or 50% higher. That sounds really significant. But you have added only one case per million people, and in total only 3 people out of a million will get the disease. If you look at it that way, then it doesn’t seem so important. Investigators can make some pretty insignificant results sound mighty important by reporting relative risk and not reporting other statistics. Thus, air pollution may play a role in these conditions, but I think the jury is still out, and we will have to await further study to be sure of how important a role.

Don’t let the fact that air pollution may play rather minor roles in causing diseases such as Parkinson’s, Alzheimers, autism, or diabetes confuse you. It is strongly linked to heart attack, stroke, chronic lung disease, and asthma, and is a significant risk factor worldwide.

This brings me to the end of this update on the Air Quality Index data for 2017. Missouri has made large strides in improving air quality. It is one of the few good news stories I get to report on. It is important that we continue to make progress, however, as air pollution is an important risk factor that causes or contributes to a great deal of death and disability around the planet.

Sources:

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