How do water systems affect indoor air quality?

Water systems affect indoor air quality whenever water becomes airborne. Humidifiers, evaporative coolers, air washers, spray taps, showers and spa pools all generate aerosol, and if that water is contaminated with legionella or other organisms, the aerosol carries them into the air people breathe. Control the water and you control that slice of the air.

A scope note before anything else

Indoor air quality is a wide subject. It covers ventilation rates, carbon dioxide build-up, damp and mould, volatile organic compounds from paints and furnishings, particulates from cooking and traffic, and more besides. Most of that has nothing to do with water systems, and this page does not pretend otherwise. What follows is deliberately narrow: the place where indoor air quality and water hygiene genuinely overlap, which is any equipment that turns water into a mist people can inhale. If your concern is the wider air-quality picture, that sits outside this site's remit.

The aerosol bridge between water and air

Legionella does its damage through the lungs, not the stomach. The bacteria multiply in water, and they reach people when that water is broken into droplets fine enough to be inhaled. Our page on how legionella spreads covers the route in detail. The same physics applies to every other organism living in a building's water: the moment water becomes aerosol, a water-system problem becomes an air-quality problem.

This is why the legionella guidance is organised the way it is. HSG274 Part 1 covers evaporative cooling systems, Part 2 covers hot and cold water systems, and Part 3 covers the other risk systems, the collection of equipment that sprays, mists or otherwise aerosolises water. Part 3 exists precisely because the aerosol is the hazard, whatever machine makes it.

The equipment that bridges air and water

• Humidifiers. Ultrasonic and spray humidifiers turn tank water directly into room aerosol, bacteria and scale included. Steam humidifiers boil the water first, which removes the microbiological risk at source. Whatever the type, the tank and its cleaning regime decide what the room breathes.
• Evaporative cooling towers and condensers. The classic large-scale aerosol source, sitting on rooftops and drift-eliminated or not, they have caused the largest legionella outbreaks on record. See cooling tower legionella control.
• Air washers and combined humidifier-cooler units. These scrub or cool building air by passing it through water, so the water loop becomes part of the air path.
• Spray taps, showers and hose-fed equipment. Every shower head is a small aerosol generator, which is why descaling shower heads is a water-safety task as well as a housekeeping one.
• Spa pools and hot tubs. The most intense aerosol source of all, covered by HSG282 and our page on legionella in hot tubs and spa pools.
• Decorative water features. Indoor fountains mist water continuously in occupied space, often with warm, stagnant reservoirs.

Legionella is not the only aerosol illness

Humidifier fever is a recognised flu-like illness that follows exposure to mist from a contaminated humidifier or air-conditioning system: fever, chills, headache and muscle aches, typically easing once the person is away from the building. It is thought to be a reaction to the mixture of organisms growing in the reservoir rather than an infection by one named species, and it has been documented in offices and other workplaces where humidifier water was left uncleaned.

The point for a duty holder is not the clinical detail, it is the direction of causation. People in a building were made unwell by the air, the air was carrying the water, and the water was carrying whatever had been allowed to grow in it. Nobody needed a named pathogen for that chain to be real. The same lesson sits behind the Pontiac fever outbreaks: aerosolised water shares whatever it contains, at scale, with everyone in the room.

What control looks like on the water side

Because the air-side risk is created on the water side, the controls live there too:

• Identify every aerosol source. The risk assessment should list each humidifier, air washer, water feature and spray-producing fitting, not just the showers and tanks. Equipment that is not on the list is not being managed.
• Design out the risk where you can. Steam humidification instead of spray, sealed chilled systems instead of evaporative cooling where the application allows.
• Keep the water clean and moving. Reservoirs and tanks need regular emptying, cleaning and disinfection, and water should not be left standing warm for days. Stagnation feeds the biofilm that shelters the organisms.
• Follow the system-specific regime. Evaporative cooling has its own HSG274 Part 1 regime of treatment, bleed-off and cleaning; spa pools have HSG282. These exist because generic good intentions do not control an aerosol source.
• Record it. Cleaning dates, dosing checks and any sampling belong in the log book, so the regime can be shown to have actually run.

Where this sits in your wider duties

The legal frame is the usual one: the Health and Safety at Work etc. Act and COSHH require the risk from legionella and other hazardous biological agents to be assessed and controlled, with ACOP L8 and HSG274 describing how. Equipment that aerosolises water is squarely inside that duty. Wider indoor-air-quality concerns, such as ventilation and damp, are managed under other guidance and are not a substitute for water-system control, just as a clean water system does nothing for a stuffy, mouldy room. The two agendas meet at the aerosol, and that meeting point deserves its own line in your records.