The post below was something I posted on a UK motoring forum 20 years ago, so I've just copied and pasted it.

https://www.honestjohn.co.uk/forum/post/index.htm?t=6781

I also mentioned car/truck windscreen washer bottles as a legionella risk, which was 8 years before the UK's HSE issued a warning about using screen wash to kill the bacteria.

https://www.bbc.co.uk/news/10293519

"The wet cooling towers are the problem, as stated by many others.

The heat removed by the cooling process is rejected at the condenser, where the refrigerant vapour is condensed into a liquid and it's latent heat of condensation is released.

In wet AC systems the condensers are cooled by water. The water is cooled at the cooling tower and recirculated. The cooling tower ponds tend to collect all the particles in the air, limescale, leaves, insects, etc, and are often at lukewarm temperatures, providing ideal conditions for the growth of many organisms. The water is intentionally sprayed into the air to cause some of it to evaporate, creating an aerosol of water droplets containing any organisms that have been cultivated in the water.

The legionella bacteria is present in water everywhere and will multiply if given favourable conditions.

The risk of infection is much greater for the elderly or infirm, so the disease has affected many hospitals, in the past. The risk wasn?t recognized until a group of elderly ex-servicemen, attending an American Legion Convention were felled by a hotel cooling tower in Philadelphia in 1976. The disease had existed prior to that, but had been regarded as yet another strain of pneumonia.

The (UK) HSE's Code of Practice on Legionnaires' disease, L8, states that a foreseeable risk exists in;

a) water systems incorporating a cooling tower;

b) water systems incorporating an evaporative condenser;

c) hot and cold water systems;

d) other systems containing water at which is likely to exceed 20 degC and which may release a spray or aerosol."