Mexican Americans who are born in the US are at least twice as likely as those born in Mexico to develop asthma, according to a new report.

Mexican Americans have the lowest rate of asthma among Hispanics in the US. However, the group includes individuals born in the US and in Mexico who might be exposed to different risk factors and, therefore, have different asthma rates.

To investigate, Dr. Fernando Holguin, from the Centers for Disease Control and Prevention in Atlanta, and colleagues analyzed data from more than 17,000 Mexican Americans who participated in the Third National Health and Nutrition Examination Survey (NHANES III) or the National Health Interview Survey (NHIS).

In both surveys, the rate of asthma among US-born Mexican Americans was around 7.5 percent, whereas the rate among their Mexican-born peers was about 2.7 percent, the investigators report in the American Journal of Respiratory and Critical Care Medicine.

On further analysis, US origin doubled the risk of asthma in NHANES III and nearly tripled the risk in NHIS, compared with Mexican origin.

The researchers also found that the rate of asthma among Mexican-born subjects increased in step with the length of time they had lived in the US.

“US-born Mexican Americans had a higher (rate) of asthma than did Mexican-born Mexican Americans, independent of access to healthcare and other” factors that might explain the relationship, the authors conclude.

“Our study suggests that both place and duration of residency should be considered when interpreting the (rate) of asthma among Mexican Americans.”

SOURCE: American Journal of Respiratory and Critical Care Medicine, January 15, 2005.