Trees filter the particles out of the air we breathe, which decreases our risk of respiratory problems, including asthma. One study found that in 2010, trees removed 17.4 million tons of air pollution across the US, which prevented 850 human deaths and 670,000 cases of acute respiratory symptoms.Scientists working in smoggy Houston say planting a 1.5-square-mile forest would make the air more breathable and could remove as much as 310 tons of ozone from the atmosphere over the course of 30 years. Plants start to exhibit damage when they are exposed to ozone levels that reach 40 parts per billion or above. Trees absorb odours and pollutant gases (nitrogen oxides, ammonia, sulphur dioxide and ozone) and filter particulates out of the air by trapping them on their leaves and bark.Trees are natural and low-cost solutions of healthy air. Best air filtration by trees occurs within 100 feet of a tree. So, street trees are associated with a lower occurrence of early childhood asthma; children living in areas with more street trees have lower prevalence of asthma. In one year, an acre of mature trees absorbs the same amount of CO2 produced when you drive your car 26,000 miles.