When to do more or less – ScienceDaily
Physical activity is important for preventing heart and vascular disease in young people as long as they don’t engage in very strenuous activity on days when air pollution levels are high, according to a national study by nearly 1.5 million people posted today (Tuesday) in the European Heart Journal.
Until now, little was known about the trade-offs between the health benefits of physical activity outdoors and the potentially harmful effects of air pollution. Previous research by the authors of the present study had investigated the issue in middle-aged people at one point, but this is the first time it has been studied in people aged 20 to 39 over a period of time. of several years. . Additionally, the researchers wanted to see what happens when people increase or decrease their physical activity over time.
Researchers from Seoul National University College of Medicine (South Korea), led by Professor Sang Min Park, examined information from the National Health Insurance Service (NHIS) in South Korea regarding 1,469,972 young Koreans living in cities, who have undergone two consecutive health examinations. during two projection periods: 2009-2010 and 2011-2012. They followed the participants from January 2013 to December 2018.
At each check-up, participants filled out a questionnaire about their physical activity over the past seven days and this information was converted into metabolic task equivalent (MET) minute units per week (MET-min / week). Participants were divided into four groups: 0, 1-499, 500-999, and 1000 or more MET-mins / week. European Society of Cardiovascular guidelines recommend that people try to do 500 to 999 minutes of MET / week and this can be achieved, for example, by running, biking or hiking for 15 to 30 minutes. minutes five times a week, or brisk walking, playing doubles tennis. or cycle slowly for 30 to 60 minutes five times a week. 
The researchers used data from the National Ambient Air Monitoring System in South Korea to calculate annual average levels of air pollution, specifically levels of small particles less than or equal to 10 or 2.5 microns in diameter, called PM10 and PM2. . .5 . The degree of exposure to air pollution was classified into two levels: low to moderate (less than 49.92 and 26.43 micrograms per cubic meter, m / m3, respectively for PM10 and PM2.5) and high ( 49.92 and 26.46 μm / m3 or more, respectively). 
Dr Seong Rae Kim, first author of the article, said, “We have found that in young adults between the ages of 20 and 39, the risk of cardiovascular disease, such as stroke and heart attack, increases. as the amount of physical activity decreased between the two screening periods in the group with low levels of exposure to air pollution.
“However, in the group with high levels of exposure to air pollution, increase the amount of physical activity to more than 1000 MET-min / week, which is more than the internationally recommended levels for the physical activity, could adversely affect cardiovascular health.This is an important finding suggesting that, unlike middle-aged people over 40, excessive physical activity may not always be beneficial for cardiovascular health in young adults. when exposed to high concentrations of air pollution.
He continued, “Ultimately, it is imperative that air pollution be improved nationally in order to maximize the health benefits of exercise in young adults. These are people who tend to exercise more than other age groups when their physical capacity is at their best. If air quality does not improve, this could lead to an increased incidence of cardiovascular disease despite the benefits for exercise health.
The researchers adjusted their results to take into account factors that might affect them, such as age, gender, household income, body mass index, smoking, and alcohol consumption. During the follow-up period, there were 8706 cardiovascular events. Among people exposed to high levels of air pollution to PM2.5, those who increased their exercise from 0 to 1000 MET-min / week or more between the two screening periods had a 33% increased risk of cardiovascular disease over time. during the follow-up period compared to those who were physically inactive and did not increase their physical activity, although this result was slightly lower than what was necessary to obtain statistical significance. This means that an additional 108 in 10,000 people could develop cardiovascular disease during the follow-up period.
Among people exposed to low to moderate levels of PM2.5, those who increased their physical activity from zero to 1000 MET-min / week or more had a 27% reduced risk of developing cardiovascular disease compared to those who remained inactive, although this result was also not entirely statistically significant. This means that 49 fewer people in 10,000 could develop cardiovascular disease during the follow-up period.
Dr Kim said: “These results are very close to statistical significance. In fact, further analysis… of our article shows that statistical significance was reached for increasing and decreasing amounts of physical activity.”
For low to moderate levels of PM10 air pollution, there was a statistically significant 38% or 22% increase in the risk of cardiovascular disease in people who started doing 1000 MET-min / week or more and then had reduced their activity to zero or 1-499 METs min / week, respectively, compared to people who maintained the same high activity level. These results were statistically significant and mean that an additional 74 and 66 in 10,000 people, respectively, would develop cardiovascular problems during the follow-up period.
Professor Sang Min Park, who led the research, said: “Overall, our results show that physical activity, particularly at the level recommended by European Society of Cardiovascular guidelines, is associated with risk. lower to develop heart and vascular disease in young adults. . However, when air pollution levels are high, exercising beyond the recommended amount can outweigh or even reverse the beneficial effects. “
The study cannot show that air pollution causes increased cardiovascular risk, only that it is associated with it. Other limitations are that there was no information on whether or not the exercise took place indoors or outdoors; participants may not remember correctly the amount of exercise they did in the seven days prior to their screening interview, although this is unlikely; PM2.5 data was only measured in three large cities; and researchers have not studied the short-term effects of exposure to air pollution.
 Examples of activity for each of the MET-min / week categories: 0 MET-min / week: no physical activity; 1-499 MET-min / week: Running, cycling, hiking etc. less than 15 minutes a day and less than 5 times a week / Brisk walking, doubles tennis, slow cycling, etc., less than 30 minutes a day and less than 5 times a week; 500-999 MET-min / week: running, cycling, hiking, etc. 15-30 minutes a day and about 5 times a week / Brisk walking, doubles tennis, slow cycling, etc., 30-60 minutes a day and about 5 times a week; More than 1000 MET-min / week: Running, cycling, hiking etc. more than 30 minutes a day and about 5 times a week / Brisk walking, doubles tennis, slow cycling, etc., more than 60 minutes a day and about 5 times a week
 A micron is a millionth of a meter.