The simple fact of growing up in a big city may not be a major factor in whether a child develops asthma, according to a new study that contradicts decades of public health assumptions about the so-called inner city asthma epidemic.

Instead, researchers at Johns Hopkins University in Baltimore found that being poor, black or Puerto Rican are the most important factors that determine a child's asthma risk.

"Our results highlight the changing face of pediatric asthma and suggest that living in an urban area is, by itself, not a risk factor for asthma," said Dr. Corinne Keet, a pediatric allergy and asthma specialist at Johns Hopkins, whose study was published on Tuesday in the Journal of Allergy and Clinical Immunology.

Although prior studies have looked at asthma rates within specific cities, no study has compared asthma rates in inner city areas across the United States, or looked at how asthma compares in other types of communities, Keet said.

To arrive at that, the researchers used national survey data on more than 23,000 children aged 6 to 17 between 2009-2011. The team looked at rates of asthma based on population figures as well as factors such as income, race and ethnicity.

After adjusting for those factors, they found no statistically significant difference in the rates of asthma between inner-city children and those who lived elsewhere.

Instead, they found Black or Puerto Rican children had far higher asthma rates, at 17 and 20 per cent, respectively, compared with white children (10 per cent), other Hispanic children (9 per cent) and Asian children (8 per cent).

Although the study did not look at why, the researchers did note that other studies suggest potential genetic and biologic causes for these racial and ethnic differences.

The team also saw wide variation by geography, with 17 per cent of children living in Northeastern cities having asthma, compared with 8 per cent in cities located in western states.

Asthma was not confined to cities. For example, asthma rates were 21 per cent in poor suburban areas of the Northeast, compared with 17 per cent in neighbouring cities.

The study did not look at factors that influence the severity of asthma, which could very well be more prevalent in cities, the authors said. That will be a subject of a follow-on study.

Asthma affects 6.8 million children, according to the US Centers for Disease Control and Prevention. – Reuters