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Legionella

CDC: home and car air-conditioning units do not use water to cool the air, so they are not a risk for Legionella growth. … and 1 more sourced claims.

Explanatory diagramRecirculation and ventilation are different
Recirculation and ventilation are different
  1. A typical split recirculates room air.
  2. Ventilation introduces outdoor air through a suitable solution.
strong evidenceCDC: home and car air-conditioning units do not use water to cool the air, so they are not a risk for Legionella growth.

CDC states verbatim that home and car air-conditioning units don't use water to cool the air, so they aren't a risk for Legionella growth. Legionella spreads via fine droplets of warm water (25–45 °C) from cooling towers, hot-water systems, hot tubs or decorative fountains. Condensate in a split is cold water that drains continuously and is not aerosolised. It is important to tell customers the truth: a dirty split can cause allergies, odour and respiratory symptoms, but 'Legionnaires' disease from the air conditioner' is a myth for household units.

Sources (1)
strong evidenceThe real Legionella risk lies with cooling towers and water systems in large buildings, which require water management plans (ASHRAE 188), not with household splits.

WHO and CDC identify cooling towers, hot-water systems, humidifiers with stagnant water and hot tubs as Legionella sources, because water there is warm, stagnant and aerosolised. For such installations ASHRAE Standard 188 requires risk-management plans. A residential split has no cooling tower and its condensate is cold and not dispersed into the air. For customers in office buildings with chillers and towers, the right advice is a water-system audit, not split sanitising as an anti-Legionella measure.

Sources (2)

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