Air pollution is a risk factor for developing noncommunicable diseases such as asthma, stroke, heart disease, lung disease, and lung cancer. Two recent studies have found that exposure to high concentrations of air pollution may also increase the risk of multiple sclerosis (MS), a neurological condition of the central nervous system (CNS) that affects more than 2.8 million people worldwide.
Air pollution is a global problem that harms our health. According to the World Health Organization, 9 out of 10 people breathe polluted air. Approximately 99 percent of the world’s population resides in areas with high levels of pollutants, especially in low- and middle-income countries in the Western Pacific and Southeast Asia.
Air pollution is defined as a combination of toxic chemicals and particulate matter that are released from gases, smoke, dust, heavy metals (such as lead, copper, and manganese), and odors that are mixed in the air we breathe. The chemicals and particulate matter cause the air to be harmful to humans, animals, and plants.
There are many causes of air pollution:
Most air pollution is caused by the burning of fossil fuels (natural fuels such as coal or gas) from:
Many health-damaging pollutants are found in polluted air, such as:
Experts have observed that a subset of particulate matter called small-diameter particles (such as PM2.5 and PM10) can enter the bloodstream, tissue, organs, and nervous system, causing short- and long-term effects on health.
The prevalence of MS varies in different parts of the world. Recent studies have found that countries with high levels of air pollution have greater MS prevalence or MS recurrence rates than countries with low air pollution levels. Several environmental and biological factors are believed to cause MS, so although air pollution may not cause MS by itself, researchers have explored whether exposure to air pollution could contribute to the development of disease.
A meta-analysis of 10 studies assessed the association between MS prevalence and relapse in six high-income countries (such as the United States, Canada, and some in Europe) with air pollution levels of PM2.5 and PM10. Researchers found that countries with high levels of air pollution had a higher prevalence of people living with MS than regions with low air pollution levels. Another meta-analysis review found that nitrogen oxides and particulate matter were also associated with increased MS relapse rates.
Another study collected data from two Turkish cities, one with relatively high levels of air pollution (home to iron and steel factories) and one with low levels of air pollution (in a rural area). Researchers found that MS prevalence was two times greater in the city with high air pollution levels than in the city with low levels of air pollution. Although their results suggest that air pollution may play a role in the development of MS, their study has a few limitations. For instance, researchers did not assess other MS risk factors in their participants, such as smoking history and vitamin D levels.
Researchers have proposed theories on the biological pathways associated with air pollution and MS. Inhaling particulate matter in the lungs may allow different toxic chemical compounds to travel into the bloodstream, directly causing an inflammatory immune response. This process may release proinflammatory substances, directly affecting the CNS and potentially worsening CNS injury.
Another theory suggests that certain genetic factors in people living with MS increase their susceptibility to infections caused by environmental factors, increasing MS relapse rates. PM10 has been found to cause infections in the airways and induce immune system responses in some people living with MS.
Overall, a strong conclusion on the association between air pollution and MS cannot be made. The current research is limited, especially in middle- and low-income countries where air pollution levels tend to be far greater than in high-income countries. More large-scale studies are needed to explore the association between pollution exposure and the development of MS.
There are many risk factors for MS, and researchers have identified a wide array of environmental factors linked to the development of MS. Some of these factors, such as smoking and vitamin D levels, may influence or be influenced by air pollution.
Smoking, which contributes to air pollution, is a risk factor for MS. In people who currently smoke, MS progresses more rapidly. Women are 1.6 times greater to develop MS if they have a history of smoking or are currently smoking compared to women who don’t smoke.
Vitamin D deficiency is another risk factor for MS. Studies have found that low vitamin D levels, which can occur due to a lack of sunlight, are associated with active lesions and increased MS relapses. Scientists have found that air pollution can absorb and block sunlight, reducing the amount of sun in an area.
Recent studies have suggested an inverse relationship between particulate matter and vitamin D levels as well as sunlight and the geographical diversity of MS. As levels of particulate matter in the air increased, the vitamin D levels of people in the area decreased. In addition, it’s been found that the rate of MS is higher in countries with low sun exposure.
Air pollution may indirectly affect vitamin D levels and, therefore, increase the likelihood of MS development or recurrence. However, the connection between vitamin D and MS is still not well understood and more research is needed.
Researchers suggest that poor air quality may be associated with autoimmune disorders such as MS. Air pollutants are linked to an increased risk of MS and disease relapse. However, the underlying cause for the association between air pollution and MS remains unknown. More research is needed to understand this association further.
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