London: The benefits of walking and cycling outweigh the negative effects on health in cities like New Delhi with high levels of air pollution, says a study which found that cycling and regular physical activity can help reduce vehicle emissions. According to the researchers, air pollution risks will not negate the health benefits of active travel in urban areas. "Even in Delhi -- one of the most polluted cities in the world -- with pollution levels 10 times those in London, people need to cycle over five hours per week before the pollution risks outweigh the health benefits," said lead study author Marko Tainio from the University of Cambridge. "We should remember that a minority of workers in the most-polluted cities, such as bike messengers, may be exposed to levels of air pollution high enough to cancel out the health benefits of physical activity," Tainio added in the paper published in the journal Preventive Medicine. The team used computer simulations to compare the risks and benefits of walking and cycling across a range of air pollution concentrations around the world. One way for people to increase their levels of physical activity is through "active travel" like example walking and cycling. The findings indicate that the main sources of air pollution in cities is transport and a shift from cars, motorbikes and buses to "active travel" would help to reduce emissions. Only one percent of cities had pollution levels high enough that the risks of air pollution could start to overcome the benefits of physical activity after half an hour of cycling every day. "This further provides support for investments in infrastructure to get people out of their cars and onto their feet or their bikes -- which can itself reduce pollution levels at the same time as supporting physical activity," stated senior study author James Woodcock from the Centre for Diet and Activity Research (CEDAR) at the University of Cambridge.