1976: Legionella Rears Its Ugly Head

In 1976, a mysterious respiratory illness struck 221 people and killed 34 after a Pennsylvania American Legion convention at Philadelphia's Bellevue-Stratford hotel. It took the CDC six months to identify the cause: a new bacterium later named Legionella pneumophila, which had grown in the hotel's cooling towers and spread through the building's air conditioning system into hospitality suites where conventioneers gathered.

This was the first documented outbreak of what's now called Legionnaires' disease, and it changed how the HVAC industry thinks about cooling towers, the large units that reject heat from air conditioning systems, often on rooftops. Legionella bacteria grow naturally in warm water between 77 and 113 degrees Fahrenheit, conditions cooling towers can provide if not properly maintained. After 1976, engineers began pushing for routine cleaning and water treatment in these systems. Formal standards took decades to arrive: ASHRAE published best-practice guidance in 2000, and a real standard requiring water management programs for qualifying buildings didn't arrive until 2015.

For homeowners, this history mainly concerns large commercial cooling towers rather than typical residential air conditioning equipment, which doesn't use the same water-based cooling process. Still, it's the reason building air conditioning is now understood as something tied to indoor air safety, not just comfort, and why commercial systems near you may have water treatment and maintenance requirements behind the scenes.