The presence of mental illness, and its severity, appear to confer an increased risk of developing asthma, researchers report.

"The relationship of asthma with psychological factors has been known for centuries, and recently there has been a resurgence of interest in this topic," Dr. Thomas H. Chun, from Brown University, Providence, Rhode Island, and colleagues point out in the medical journal Chest.

The researchers reexamined the relationship in a representative sample of the US population, using data from more than 355,000 subjects who participated in a 2006 survey. To the authors' knowledge, this is the largest investigation of the asthma-mental health connection.

Self-reported poor mental health was directly related to the odds of currently having asthma, the team found. Moreover, as the number of self-reported days of poor mental health increased, so did the likelihood of having asthma.

Specifically, asthma affected 6.2 percent of respondents with no mental health issues, 8.7 percent of those with 1-7 days of poor mental health, climbing stepwise to 21.6 percent of those with 22 days or more of poor mental health.

However, comments Dr. David M. Mannino, from the University of Kentucky, Lexington, in an editorial, "In the end, this analysis does not really differentiate whether the observed nexus is merely a linkage between two common problems, or indicative of a pathway where one problem is central to the development of the other."