"Why walking is good for you ... even in the smog. Health benefits of a stroll found to outweigh harm caused by chemicals and dust pumped out by traffic," says the Mail Online.
The report in question was carried out to see whether the harm caused by exposure to air pollution outweighs the benefit of doing exercise.
The study used computer modelling and found that the pollution level needed for the harms and benefits of exercise to become equal, the "tipping point", was only present in 1% of cities, according to the World Health Organization (WHO).
In an average city, physical exercise will remain beneficial for up to seven hours a day on a bicycle or walking for 16 hours. But in the most polluted cities, such as Delhi, this became as low as 30 minutes a day for cycling and 90 minutes of walking.
Although findings from these types of computer model studies have to be interpreted carefully, their findings can be quite accurate as long as the data used is accurate.
Keeping active can reduce your risk of illness such as heart disease, stroke and type 2 diabetes. This study suggests that in an urban environment you are unlikely to be putting your health at risk by exercising outdoors.
The study was carried out by researchers from a number of institutions including the Centre for Diet and Activity research at the University of Cambridge and the Centre for Environmental Policy at Imperial College London.
Funding was provided by the British Heart Foundation, Cancer Research UK, the Economic and Social Research Council, the Medical Research Council, the National Institute for Health Research, and the Wellcome Trust.
The study was published in the peer-reviewed journal: Preventative Medicine.
The results of the study were presented accurately in the media.
This was a health impact modelling study which assessed the risk-benefit balance between exposure to pollution through outdoor physical activity and the health benefits of exercise itself.
Previous work on the topic focused on high income countries with low pollution levels, but health risks are thought to rise with increased exposure to air pollution, this was investigated in the study.
Modelling studies are useful for investigating these scenarios, however as it is only a model it may never be true to life.
The researchers carried out computer simulations, using data from epidemiological studies and meta-analyses, to assess exposure to air pollution through physical activity and the associated health risks around the world.
Walking and cycling were considered in the simulations and the concentration of pollution established that was required to reach the "tipping point", where the risk from pollution and the health benefit from exercise are equal, and the "break-even" point, beyond which any time spent exercising would cause adverse health effects.
The researchers found that if cycling for 30 minutes, a pollution concentration (PM2.5) of 95 microgram/m3 (seen in less than 1% of cites according to the WHO Ambient Air Pollution Database) is required to meet the tipping point.
The breaking point is reached at a concentration of 160 microgram/m3.
For an average urban pollution concentration the tipping point would be reached after seven hours of cycling per day.
If walking for 30 minutes, the tipping and breaking points would be at a concentration above 200 microgram/m3, meaning in an average urban area, the tipping point would be reached after 16 hours of walking per day.
Highly polluted cities, such as Delhi, had low tipping and breaking points, these were 30 and 45 minutes of cycling per day.
Tipping points for the most polluted cities (44 microgram/m3 to 153 microgram/m3) varied between 30 and 120 minutes per day for cycling, and 90 minutes to 6 hours 15 minutes per day for walking.
The researchers conclude: "The benefits from active travel generally outweigh health risks from air pollution and therefore should be further encouraged.
"When weighing long-term health benefits from PA [physical activity] against possible risks from increased exposure to air pollution, our calculations show that promoting cycling and walking is justified in the vast majority of settings, and only in a small number of cities with the highest PM2.5 concentration in the world cycling could lead to increase in risk."
This modelling study aimed to assess exposure to air pollution through physical activity and the associated health risks around the world.
The study found the background pollution level required to reach the tipping point is only present in less than 1% of cities, according to the WHO.
In an average city physical exercise will remain beneficial up to seven hours a day for cycling or 16 hours for walking.
In highly polluted areas this became as low as 30 minutes a day for cycling and 90 minutes of walking.
The main limitation of this study is that it is only a model and we do not know how true to life the findings are. But these studies can be quite accurate if representative data is used in the model.
The results will reassure those concerned about the effects of pollution on their health.
The study did not describe differences between children, adults and older adults or those with health conditions, as the tipping point may be different amongst these groups.
Keeping active can reduce your risk of illness such as heart disease, stroke and type 2 diabetes.
It is recommended that to stay healthy you should be active daily, this can be moderate activity for at least 150 minutes per week, 75 minutes of vigorous exercise or a mixture of both.
The findings of this study suggest that in an urban environment it is unlikely that you are putting your health at risk by exercising outdoors.