All cooling towers in New York City must be inspected quarterly for Legionella bacteria and disinfected if found to be contaminated, under legislation signed Tuesday by Mayor Bill de Blasio.

The law was motivated by the worst outbreak of Legionnaires' disease in city history. The flare-up, concentrated in the South Bronx, has killed 12 people and sickened 115 others since the first case was diagnosed last month. One or more of the towers, which help regulate heating, ventilation and air-conditioning systems in many modern buildings, is suspected to have caused the outbreak.

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"We must minimize the odds of any future outbreaks," de Blasio said, moments before signing the bill in the City Hall Blue Room. "When it comes to New Yorkers' health, we won't take any chances."

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De Blasio said epidemiologists believe that latest outbreak is contained. There have been no new diagnoses since Aug. 3, though since Saturday the number of known cases reported to the health department has increased by three.

The administration called the law, Introduction 866, the nation's first to mandate cooling tower maintenance.

Nationally, about 60 percent of patients who have had Legionnaires' disease were infected via cooling towers, said Dr. Mary T. Bassett, the city health commissioner. The standards for decontamination, whether a building owner must hire an outside firm and other specifics will be part of rules the city is in the process of drafting, said Rick Chandler, city buildings commissioner.

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The law also requires building owners to register their towers with the city. There is currently no precise count of such towers, though officials have so far identified thousands using aerial photography and other patchwork methods.