A 20-year trend of rising asthma rates among American children appears to have started decreasing during the last two years, show new findings published in the journal Pediatrics by researchers at the National Center for Health Statistics (NCHS). But experts caution that diagnosis rates for the respiratory condition among poor kids and children in the South are still on the rise, UPI reports.

For the study, researchers at the NCHS reviewed survey information from 2001 to 2013. Scientists reported that childhood asthma increased between 2001 and 2009, then plateaued for four years before beginning to decline overall in 2013. Researchers noted that young African-American and Mexican children, and kids in the Midwest, saw the biggest declines in asthma frequency. But the report also noted there was no change in asthma prevalence among white or Puerto Rican children, or children living in the northeast and western regions of the country. What’s more, the findings showed asthma rates seemed to have increased overall among kids aged 10 to 17, poor children, and youngsters living in the South.

Scientists said they’re not entirely sure why asthma rates leveled off overall. But they noted several environmental factors, such as mold, tobacco, mildew, dust, cockroaches and smog, could be putting poor kids at a bigger risk of the condition. In addition, researchers suggested that the stress of poverty might also affect asthma rates.