The link between air pollution and autoimmune diseases just got a little clearer thanks to a massive study that dove into the health records of over 6 million Canadians.

Led by public health researcher Naizhuo Zhao of McGill University, the study found that long-term exposure to air pollutants was associated with a slightly increased risk of developing lupus, Sjogren's syndrome, scleroderma, and other less common systemic autoimmune diseases.

These conditions, each affecting roughly 1 in 10,000 people, often get overshadowed by autoimmune diseases such as rheumatoid arthritis, type 1 diabetes, and multiple sclerosis (MS), which are more prevalent.

But research shows that autoimmune diseases are on the rise worldwide, and with roughly 80 autoimmune conditions together affecting millions of people globally, they exact a terrible toll.

"In recent years, growing attention has been paid to the role of environmental factors to help explain the development of these diseases," researchers explained in 2019 in a review of the emerging role of air pollution in autoimmune diseases, led by epidemiologist Chan-Na Zhao of China's Anhui Medical University.

In autoimmune diseases, the body's immune system goes awry and, depending on the disease, immune cells can mistakenly assault cells throughout the body: in the skin, gut, organs, nerve cells, joints, or connective tissues.

How air pollution meddles with body systems (aside from irritating the lungs) or acts on underlying genetic factors is not entirely clear. But it's thought that inhaling air pollutants over time triggers systemic inflammation throughout the body, which may lead to or exacerbate a number of autoimmune diseases.

For instance, previous studies have shown that exposure to air pollution triggers flare-ups of rheumatoid arthritis and may contribute to multiple sclerosis.

In the new study, published in Arthritis Research & Therapy, Naizhuo Zhao and fellow researchers at McGill University analyzed the health records of more than 6 million Quebec residents aged 18 years and over, half of whom were women.

Inferring people's air pollution exposure from their postcode, they found a positive association between autoimmune disease onset and particulate matter in air pollution – specifically, fine particles sized 2.5 micrometers or less, called PM2.5, which have before been linked to cardiac and chronic lung conditions.

People in the study had health records spanning at least four years (on average 10 years), during which time around 32,200 people were diagnosed with a systemic autoimmune disease.

The researchers adjusted for age, sex, socioeconomic status, and whether people lived in urban or rural areas. Controlling for smoking habits did not alter the findings.

While the increased risk of autoimmune disease with air pollution exposure was slight, what's concerning is that Quebec generally has air pollution levels well below the air quality standards enforced by the Canadian government. Which is another way of saying even 'low' levels of air pollution seem to be harmful to health.

More studies are needed to better understand the interplay between different forms of air pollution, particularly ozone, a harmful pollutant, and autoimmune diseases, the researchers say.

No clear association was found between ozone exposure and the onset of autoimmune diseases in this study. Plus, the analysis didn't include data on other gases, such as carbon monoxide or sulfur dioxide, which come from burning fossil fuels.

"We should [also] note that, in using administrative health data, we are only studying individuals who have presented for (and received) health care," the team writes in their paper.

It's important to remember too that a host of other factors are entangled in autoimmune diseases, including genetics, diets of ultra-processed foods, and common viral infections (in the case of MS). The biological underpinnings of sex differences in autoimmune diseases are also coming into focus.

But given that air pollution blankets whole cities, targeting it could have huge health benefits. Recent estimates suggest millions of early deaths could be prevented worldwide each year if we phased out fossil fuels, and made changes to transport and heavy industry to reduce air pollution.

Because it's not just the risk of autoimmune diseases that air pollution worsens. Another study also published this week in PLOS Global Public Health shows poor air quality raises the risk of developing two or more chronic diseases.

The analysis, of roughly 19,000 older adults surveyed about their health over four years, in conjunction with historical satellite data of PM2.5 levels in 125 Chinese cities dating back 15 years, comes from population health graduate student Kai Hu and colleagues at the University of St. Andrews in the UK.

Here, the researchers found that amongst older adults aged 45 years and over, a higher cumulative exposure to PM2.5 – of as little as 1 microgram per cubic meter (μg/m3 or 0.001 parts per million) – was associated with a greater likelihood of developing multiple chronic diseases, such as high blood pressure and lung disease.

Older folks do tend to be more susceptible to environmental risk factors than younger people. But whatever your age, these are health risks we could do without – and we can, by cutting air pollution.

The two new studies are published in Arthritis Research & Therapy and PLOS Global Public Health.