Childhood asthma rates appear to have stopped rising among many U.S. groups, but not among the poorest kids or children ages 10 and older, a government study suggests.

Overall, asthma prevalence among Americans under 18 had been rising for decades, until it peaked at 9.7 percent in 2009. Then it held steady until 2013, when it dropped to 8.3 percent, from 9.3 percent the previous year, researchers reported in the journal Pediatrics.

“International data on asthma prevalence over time shows that trends appear to be leveling off in many countries, and suggests that the trend in the United States seems to be following a general pattern,” said the lead study author, Lara Akinbami of the Centers for Disease Control and Prevention’s National Center for Health Statistics.

Part of the complexity stems from variations in rates of asthma based on age, income, region, or race and ethnicity, the study found.

There was no change in asthma prevalence from 2001 to 2013 for white or Puerto Rican children or for kids living in the Northeast or West. In the same period, prevalence rose for kids ages 10 to 17, poor children and residents of the South.

Disparities in asthma between white and black children stopped increasing, and Puerto Rican kids continued to have the highest prevalence.

For low-income children in particular, factors such as tobacco exposure, poor indoor air quality, and indoor dust-mite and cockroach exposure may make asthma more likely, said Avni Joshi of the Mayo Clinic.