Congresswoman for NY-14 which includes parts of Bronx and Queens, Ocasio-Cortez was talking to the congress, she claimed the following:
Watch every second of this… @AOC is so incredibly spot on. https://t.co/ESP4dC5TTo
“[Children] in the south Bronx…are suffering from the highest rates of childhood asthma in the country.”
Asthma rates have been a problem in the South Bronx for a long time many decades . Rates of asthma are higher among members of minority groups such as blacks and Puerto Ricans[3,4], groups that make up a majority of the population of the South Bronx.
One study of data from 2002 to 2003 found that childhood asthma rates in the South Bronx were “eight times higher than the national average” 
Chronic asthma among children on a national level in 2016 was 8.3% according to the CDC.  A statement released by NYC Department of Health put Bronx asthma rates at 8.1%, highest in New York. 
A multi-state study of 1772 children ages 5–11 yrs published in 2009 found that “Bronx children were significantly more likely to be sensitized to reported aeroallergens in their homes than were children from the other sites”.  While this study did not determine that children living in the Bronx had higher rates of asthma it did conclude “poorer asthma outcomes” in Bronx children.