WebMD, an online medical news source, recently released a list of the ten U.S. cities with the most frequent cases of asthma.

To create the list, researchers used the following criteria: air pollution levels, pollen levels, number of reported asthma attacks and cases, whether bans on smoking in public locations are in place, reported use of “rescue inhalers” to treat asthma attacks, and poverty rates (asthma rates are invariably correlated with high poverty rates).

“Early on, we found that many conventional cleaning chemicals can trigger asthma attacks among school children,” says Stephen Ashkin, President of The Ashkin Group. “This is why so many schools were early pioneers in [adopting] Green Cleaning products and practices.”

According to WebMD, these ten cities have the most prevalent asthma rates:

No. 10: Allentown, PA
No. 9: McAllen, TX
No. 8: Oklahoma City, OK
No. 7: St. Louis, MO
No. 6: Harford, CO
No. 5: Chattanooga, TN
No 4: Pittsburgh, PN
No. 3: Knoxville, TN
No 2. New Haven, CO
No 1: Memphis, TN

The study also points out that recent statistics indicate asthma causes more than 3,300 deaths annually in the U.S. and is a factor in another 7,000.

“And it is still the most common health reason for repeated absences from schools,” adds Ashkin, “which tells me we still have a ways to go in cleaning and maintaining our schools in a healthier, more environmentally preferable manner.”