Darlene Summerville, carrying daughter Ka-mia, and other residents call Lemmon Street “the Forgetabout Neighborhood.” (Doug Kapustin for Kaiser Health News) BALTIMORE — Keyonta Parnell has had asthma most of his young life, but it wasn’t until his family moved to the 140-year-old house here on Lemmon Street two years ago that he became one of the health-care system’s frequent customers.  “I call 911 so much since I’ve been living here, they know my name,” said the 9-year-old’s mother, Darlene Summerville, who calls the emergency medical system her “best friend.” Summerville and her family live in the worst asthma hot spot in Baltimore: Zip code 21223, where decrepit houses, rodents and bugs trigger the disease and where few community doctors work to prevent asthma emergencies. Residents of this area visit hospitals for asthma flare-ups at more than four times the rate of people from the city’s wealthier neighborhoods, according to data analyzed by Kaiser Health News and the University of Maryland’s Capital News Service.  Baltimore paramedic crews make more asthma-related visits per capita in 21223 than anywhere else in the city, according to fire department records. It is the second-most-common Zip code among patients hospitalized for asthma, which, when addressed properly, should never require emergency visits or hospitalization. The supreme irony of the localized epidemic is that Keyonta’s neighborhood in southwest Baltimore is in the shadow of prestigious medical centers — Johns Hopkins, whose researchers are international experts on asthma prevention, and the University of Maryland Medical Center (UMMC).  Both receive massive tax breaks in return for providing “community benefit,” a poorly defined federal requirement that they serve their neighborhoods. Under Maryland’s ambitious effort to control medical costs, both are supposed to try to improve residents’ health outside the hospital and prevent admissions.  But like hospitals across the country, the institutions have done little to address the root causes of asthma. The perverse incentives of the health-care payment system have long made it far more lucrative to treat severe, dangerous asthma attacks