For the first time, environmental air pollution emerges as a leading risk factor for stroke worldwide

Originally published by The Lancet Neurology on June 9, 2016

Three-quarters of strokes worldwide could be prevented by addressing behavioral risk factors such as smoking, poor diet, and low physical activity

Environmental air pollution has emerged as one of the leading risk factors for stroke worldwide, associated with a third of the global burden of stroke in 2013, according to a new study published in The Lancet Neurology.

The findings, from an analysis of global trends of risk factors for stroke between 1990 and 2013, also show that over 90% of the global burden of stroke is linked to modifiable risk factors, most of which (74%) are behavioral risk factors such as smoking, poor diet, and low physical activity. The authors estimate that control of these risk factors could prevent about three-quarters of all strokes.

The study is the first to analyze the global risk factors for stroke in such detail, especially in relation to stroke burden on global, regional, and national levels. The researchers used data from the Global Burden of Disease study to estimate the disease burden of stroke associated with 17 risk factors in 188 countries. They estimated the population attributable fraction (PAF) – i.e., the attributable risk of stroke-related disability-adjusted life years (DALYs) – associated with environmental, occupational, behavioral, physiological, and metabolic risk factors.

Every year, approximately 15 million people worldwide suffer a stroke – of these, nearly 6 million die and 5 million are left with permanent disability [1]. Stroke is the second leading cause of disability, after ischemic heart disease [2]. Disability may include loss of vision and/or speech, paralysis, and confusion.

Globally, the 10 leading risk factors for stroke were high blood pressure, diet low in fruit, high body mass index (BMI), diet high in sodium, smoking, diet low in vegetables, environmental air pollution, household pollution from solid fuels, diet low in whole grains, and high blood sugar (figure 2). One-third (29.2%) of global disability associated with stroke is linked to environmental air pollution – and this is especially high in developing countries (33.7% vs 10.2% in developed countries).

“A striking finding of our study is the unexpectedly high proportion of stroke burden attributable to environmental air pollution, especially in developing countries. Smoking, poor diet, and low physical activity are some of the major risk factors for stroke worldwide, and controlling these behavioral risk factors could prevent about three-quarters of strokes worldwide” says lead author Professor Valery L Feigin, of Auckland University of Technology, New Zealand. [3]

“Our findings are important for helping national governments and international agencies to develop and prioritize public health programs and policies. Governments have the power and responsibility to influence these risk factors through legislation and taxation of tobacco, alcohol, salt, sugar, or saturated fat content, while health service providers have the responsibility to check and treat risk factors such as high blood pressure,” he says. [3]

“Taxation has been proven to be the most effective strategy in reducing exposure to smoking and excessive intake of salt, sugar, and alcohol. If these risks take a toll on our health, and taxation is the best way to reduce exposure to these risks, it logically follows that governments should introduce such taxation and reinvest the resulting revenue back into the health of the population by funding much needed preventative programs and research in primary prevention and health. All it takes is recognition of the urgent need to improve primary prevention, and the good will of the governments to act,” says Professor Feigin. [3]

The relative importance of risk factors varied depending on age group, country and region:

• Household air pollution was a more important risk factor for stroke in Central, Eastern, and Western sub-Saharan Africa and South Asia (ranked third), compared to North America, Central, Eastern and Western Europe (where it was not in the top 10 risk factors) (paper, figure 2).
• Low physical activity was a much greater risk factor for stroke among adults over 70 than among adults aged 15-69 (Appx, table 2).
• Globally, the risk factor that was most reduced between 1990 and 2013 was secondhand smoke (31% reduction in stroke-related DALYs). The greatest reduction was in developed countries (Appx, table 4), but the contribution of secondhand smoke to global stroke burden still remains noticeable at 3.1% for 15-49 year olds, especially in developing countries where it reaches 3.2% (Appx, table 2). 
• The risk factor that was most increased was a diet high in sugar-sweetened beverages (63.1% increase in stroke-related DALYs). The greatest increase was in developed countries (Appx, table 4), but the contribution to stroke burden remains low at 1.6% for 15-49 year olds (Appx, table 2).  
• Air pollution, environmental risks, tobacco smoke, high blood pressure, and dietary risks were more important risk factors for stroke in developing countries compared to developed countries.
• Low physical activity was a more important risk factor for stroke in developed countries compared to developing countries.

The authors say that because of a lack of data, they could not include some important risk factors for stroke such as atrial fibrillation, substance abuse, or other health conditions. They were also unable to account for patterns of some risk factors such as levels of smoking, BMI level, or underlying genetic risk factors. The data do not differentiate between ischemic and hemorrhagic strokes, but the authors say that while the risk factors for different types of stroke may vary slightly at the individual level, global, regional, and national policies tend to look at the overall risk of stroke.  

The study also provides information on the contribution of all 17 risk factors for stroke for 188 countries; for example, the top five risk factors for stroke in the following countries were:

• UK & USA: high blood pressure, high BMI, diet low in fruit, diet low in vegetables, smoking (Appx, table 7 and 8).
• India: high blood pressure, diet low in fruit, household air pollution, diet low in vegetables, diet high in sodium (Appx, table 7 and 8).
• China: high blood pressure, diet low in fruit, diet high in sodium, smoking, environmental air pollution (Appx, table 7 and 8).

Download the study here: http://www.thelancet.com/journals/laneur/article/PIIS1474-4422(16)30073-4/abstract

[1] http://www.world-heart-federation.org/cardiovascular-health/stroke/

[2] Christopher Murray et al. Global, regional, and national disability-adjusted life years (DALYs) for 306 diseases and injuries and healthy life expectancy (HALE) for 188 countries, 1990–2013: quantifying the epidemiological transition.  The Lancet 2015; 386 (10009), pp. 2145-2191