"The importance of knowing this for people with COPD and their families is that indoor smoking is under our control," lead study author Dr. Liesl M. Osman told Reuters Health.

"We can immediately very much improve air quality by stopping smoking in the home," said Osman, a senior research fellow at the University of Aberdeen in Scotland.

COPD includes emphysema and chronic bronchitis, two serious lung diseases frequently caused by smoking. Breathing in other irritants, such as polluted outdoor air, can contribute to or exacerbate COPD, but less is known about the importance of household air pollution.

To investigate, Osman's team measured air quality in the homes of 148 Scottish adults with severe COPD. They also questioned them about their respiratory symptoms and smoking habits.

Thirty-nine percent of the subjects were smokers and 49 percent lived in a household where someone smoked.

In general, the researchers found, patients' homes had high levels of particulate matter -- the fine airborne particles that constitute pollution. Smokers' homes had especially high concentrations.

Homes with the highest levels of particulate matter exceeded the maximum levels recommended by the EPA by about four-fold. Overall, the greater the level of particulate matter was, the worse patients' COPD symptoms were.

A number of factors, including outdoor air pollution, can affect indoor air quality, according to Osman. However, the current findings show that smoking is "a big contributor," she noted.

"Our study shows that environmental tobacco smoke exposure worsens symptoms among people with COPD," Osman said. It's reasonable to assume that poor indoor air quality also worsens the long-term prognosis for COPD patients, she added, but long-range studies are needed to confirm this.