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.”
Fact check: Mostly True
Asthma rates have been a problem in the South Bronx for 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”  Other studies have found that “Pediatric asthma prevalence rates in Bronx children far exceed national pediatric asthma rates,” and “are more likely to die of asthma than other US children” 
It is clear from the data that the South Bronx has bad asthma rates compared to the US as a whole, either for children or for people in general. The South Bronx also has among the worst adult asthma rates in New York City.
How does South Bronx compares to other asthma hot spots?
The Asthma and Allergy Foundation of America (AAFA) puts New York City as number 20 on its list of the top “Asthma Capitals” . On the one hand, this means that there are other urban areas in the United States with high rates of asthma, possibly worse than the Bronx. On the other hand, the South Bronx has some of the worst asthma rates in a city with one of the worst overall rates in the country.
Because the South Bronx is a national childhood asthma hotspot, but there is no region with clearly the highest rates in the country, we rate this claim as mostly true.