Researchers at The Johns Hopkins University said that nitrogen dioxide, compared the frequency and intensity of coughing, wheezing, shortness of breath and chest tightness to nitrogen dioxide levels inside the inner-city homes of 150 Baltimore children 2 to 6 years old.

A report, published in the Environmental Health Perspectives, found asthma flare-ups were directly related to high concentrations of nitrogen dioxide in the inner city homes studied.

The researchers said that across the board the pollutant worsened day and night symptoms. Each 20-point increase in nitrogen dioxide levels led to 10 percent more days of cough and 15 percent more days with limited speech due to wheezing.

"Because using stoves as heat sources is a hallmark of urban poverty, our study tellingly points to how profound and direct the effects of purely social and environmental factors can be on a child's health," lead researcher Dr. Nadia Hansel said in a statement.

Nitrogen dioxide, an irritating and toxic form of nitrogen oxide gas, is most prevalent in industrial zones, but also found at higher levels in poor homes with unvented gas stoves.