A new report says that trees planted in the cities may be worsening air pollution rather than reducing it.

It is a common belief that trees in urban areas have a positive impact on the environment because they help in taking up carbon dioxide emissions and prevent floods. However, in the draft report of a new study, the authors said: "some people believe that trees reduce air pollution but this is not always true, and it is important to address this misconception."

The authors added that "their effect is dependent on factors including street design, number and sitting of trees, species and canopy density, time of year and wind direction relative to the street."

The authors elaborated that the said factors contribute to slowing and trapping water currents down while providing protection to infrastructures above trees. However, they added that the presence of plants serve as particulates "sinks" that may pin down volatile organic compounds affecting air quality. The National Institute for Health and Care Excellence (NICE) explained that "Leaves and branches slow air currents causing pollutants to settle. They may also act as sinks for particulates and chemicals that may have direct or indirect effects in air quality. Air quality may deteriorate at street level near vehicles."

The guidance draft made by NICE aims to help UK local units to address the problem. The authors, however, state that "it is also important to recognize the benefits of properly selected, sited and managed trees."

Air pollution is a global problem and has been an issue in the cities of Beijing and Mumbai. In London, air pollution has surpassed the annual safe limit standards of the European Union. Unclean air causes illnesses and thousands of deaths globally.

The report saying that trees in the cities worsen pollution underscored that it does not suggest that all should be cut down.

There are other ways to reduce emissions said NICE director for the center of clinical practice, Mark Baker. "We can take steps now to encourage people to walk or cycle rather than drive but these efforts will be futile if we do not have a long-term plan to improve air quality," he noted.