Ventilation, Air Purifier, Humidifier and CO₂ Monitor: What Each One Does

Opening a window, running an air purifier and switching on a humidifier often get lumped together as ways to get “fresher air”. They are not interchangeable. Ventilation swaps indoor air for outdoor air, a purifier filters particles out of the same indoor air, and a humidifier or dehumidifier changes only how much water that air holds. The US Environmental Protection Agency puts the strategies in order: remove the source, ventilate, then clean the air; a purifier adds to the first two and does not replace them (EPA).

What is happening

Ventilation is the only one that replaces air. Outdoor air gets in through gaps in the building, open windows and doors, and fans; wind and the indoor–outdoor temperature difference drive the first two (EPA). The US ventilation standard counts both floor area, because materials and furniture give off substances all the time, and occupants, who add CO₂ and moisture (ASHRAE 62.2, as summarised by the EPA).

A purifier pulls room air through a filter. True HEPA is designed to catch at least 99.97% of particles 0.3 micrometres across, the hardest size to catch (EPA). Capacity matters as much: the US rule of thumb from the makers' association AHAM, quoted by the EPA, is a clean air delivery rate (CADR) for smoke in cubic feet per minute of at least two-thirds of the floor area in square feet, roughly 12 m³/h per square metre at a normal ceiling height. Without an activated-carbon filter a purifier does not remove gases. Avoid machines that make ozone on purpose; ionisers and “plasma” devices can release it as a by-product. Ozone irritates the lungs and, at levels below health limits, does little to clean the air (EPA).

A humidifier or dehumidifier changes only the water in the air. A humidifier with stale water in the tank becomes a source of bacteria, mould and mineral dust itself (EPA). Whether you need either is a question for a hygrometer; how to run a humidifier safely is on the humidifier page.

A CO₂ monitor is not an overall air quality score. It shows how much outdoor air each person in the room is getting, because people breathe CO₂ out along with everything else they emit (ASHRAE). It does not detect carbon monoxide, particles or fumes, and a purifier does not lower it, so a good reading says nothing about smoke or dust (HSE, UK). Sensors labelled “eCO2” or “CO2eq” estimate the figure from other gases instead of measuring CO₂ (Sensirion datasheet); the UK Health and Safety Executive recommends NDIR (infrared) monitors. What the readings mean and how long to air rooms is covered in airing out your home.

What to do

• Source and air first: extractor fans that vent outside while cooking and showering, and regular airing (EPA).
• A purifier is for particles: true HEPA, sized to the room, a carbon filter for odours, no ozone generators or ionisers. Change filters as the maker says: air filters on a calendar.
• A humidifier only if a hygrometer shows dry air, and only a clean one.
• A CO₂ monitor is a ventilation reminder, not a safety alarm.
• Carbon monoxide needs its own detector. CO has no colour or smell and comes from faulty boilers, blocked flues, generators and engines (CDC). See carbon monoxide and testing smoke and CO detectors.

When this is not the case

• Wildfire smoke or heavy smog. An open window lets indoors become about as smoky as outdoors. The EPA (US) advises closing windows and doors, one clean room with a purifier and air conditioning on recirculate.
• High pollen. The NHS (UK) advises keeping windows and doors shut as much as possible and using a vacuum with a HEPA filter.
• A stuffy bedroom at night. Small studies found much lower night-time CO₂ with the window open and somewhat better sleep, but the groups were tiny (Strøm-Tejsen et al.). Ventilate through a window, trickle vent or extractor, not an open door: a closed door slows a fire, see sleep with the bedroom door closed.
• Someone at home is ill: when someone is sick at home, how colds spread.
• Damp and mould. Neither a purifier nor airing fixes a leak or a cold wall: checking for mould.

Sources

• EPA — Improving indoor air quality
• EPA — Guide to air cleaners in the home
• EPA — What is a HEPA filter?
• EPA — Ozone generators sold as air cleaners
• EPA — How much ventilation do I need?
• EPA — Use and care of home humidifiers
• EPA — Cleaner air room at home
• ASHRAE — Position document on indoor carbon dioxide
• HSE — Using CO2 monitors
• Sensirion — SGP30 datasheet
• CDC — Carbon monoxide poisoning basics
• NHS — Hay fever
• Strøm-Tejsen et al., Indoor Air, 2016

Checked: 24 September 2026