Ventilation and Air Purifiers: What Actually Reduces COVID-19 Indoors

Of all the precautions people took during the pandemic, ventilation was probably the least visible and among the more useful. It requires no purchase, no behaviour change from anyone else in the room and no ongoing effort beyond opening a window. It is also widely misunderstood, particularly the part where an expensive appliance is assumed to do the same job as fresh air.

Why indoor air matters more than surfaces

Early in the pandemic a great deal of attention went on wiping down shopping and disinfecting door handles. Understanding shifted substantially as evidence accumulated that the virus spreads mainly through respiratory particles that a person breathes out, which can remain suspended in the air of an enclosed space for a considerable time. That shift is what moved ventilation from an afterthought to one of the central measures, since diluting and removing that air is a direct way of reducing how much of it anyone else breathes in.

What ventilation actually means

Ventilation is simply the replacement of indoor air with outdoor air. That can happen naturally, through open windows, doors and vents, or mechanically, through extract fans and building systems that draw air in from outside. The distinction that matters is between systems that bring in fresh outdoor air and systems that only move existing indoor air around. A ceiling fan or a portable fan on its own does not ventilate a room, because it recirculates the same air rather than replacing it, though it can help direct air towards an open window.

How much fresh air is enough?

UK government guidance on living safely with respiratory infections takes a practical rather than a precise approach, which is sensible given how much buildings differ. Its position is that opening a door or a window, even for a few minutes at a time, helps remove older stale air that could contain virus particles, that trickle vents are useful for bringing a little fresh air in constantly and that the more fresh air you bring inside, the quicker any virus is cleared from the room. It also notes that ventilation matters most when someone in the household is actually unwell, which is a useful way of deciding when it is worth being deliberate about it.

Using carbon dioxide as a rough guide

Carbon dioxide monitors became popular during the pandemic because they offer an indirect signal. People breathe out carbon dioxide, so in an occupied room a rising reading suggests that exhaled air is accumulating faster than it is being replaced. It is a proxy rather than a measurement of virus. It says nothing about whether anyone in the room is actually infectious, but it does give a rough sense of whether a space is being ventilated adequately for the number of people in it. In practice the most useful thing a monitor does is reveal which rooms in a building are much worse than they feel.

Where air cleaners fit in

A portable air cleaner filters particles out of the air in a room. It does not bring in fresh air, so it is a supplement to ventilation rather than a replacement for it. Where it earns its place is in rooms that genuinely cannot be ventilated well, such as internal rooms with no openable window, or spaces where opening windows is impractical for long stretches in winter. Used in that role, with a suitably sized unit for the room, it can meaningfully reduce airborne particle levels. Used as a reason not to open a window in a room that has one, it is largely wasted money.

What to look for in an air cleaner

The specification that matters most is whether the unit uses a HEPA filter, since that is what captures fine airborne particles. The second is the unit's clean air delivery rate relative to the size of the room, because a device rated for a small bedroom will do very little in an open-plan office. The third is noise, which sounds trivial but decides whether the thing gets run on a useful setting or turned down to near-silence where it moves almost no air. Filters also need replacing on schedule, since a neglected filter is a slowly degrading one.

Devices worth being cautious about

Not everything sold as an air purifier is a filter. Some units work by generating ozone or by releasing ions, both of which carry a different risk profile. Ozone is a respiratory irritant, which makes deliberately generating it in an occupied room a questionable trade against an uncertain benefit. Ultraviolet devices exist in legitimate professional forms, but consumer versions vary widely in whether they expose air to the light for long enough to do anything. When in doubt, a straightforward HEPA unit from a reputable manufacturer is the option with the least to go wrong.

Ventilating in winter without freezing

The obvious objection to ventilation advice is that it is written as though it were always June. In cold weather the practical compromise is short bursts rather than constant airflow: opening windows wide for a few minutes clears a room's air far more effectively than leaving one cracked open for hours. It also cools the room's surfaces much less, so the temperature recovers quickly. Ventilating rooms while they are unoccupied, such as airing a meeting room between bookings, achieves much of the same benefit without anyone sitting in a draught.

What this looks like in practice

At home, the highest-value moments are when someone in the household is unwell or when visitors are round for an extended period. Airing the rooms in use, keeping the door of an unwell person's room closed with their window open and running extract fans in kitchens and bathrooms covers most of it. In workplaces, the equivalent is knowing whether the building's system supplies fresh air or recirculates it, which the facilities team will know. In schools, ventilation is now largely a matter of established routine rather than emergency measure. None of this requires a return to pandemic-era behaviour. It applies just as well to flu and other respiratory infections.

A quick recap

COVID-19 spreads mainly through the air indoors, so replacing indoor air with outdoor air is one of the more effective and least demanding precautions available. Short bursts of wide-open windows work well, particularly in winter. Carbon dioxide monitors give a rough indication of whether a room is being ventilated adequately. HEPA air cleaners are a reasonable supplement in rooms that cannot be ventilated, sized appropriately for the space, while ozone-generating devices are best avoided. Fans that only recirculate air do not ventilate anything.

Frequently asked questions

Is an air purifier as good as opening a window?

No, because a filter removes particles but does not bring in fresh outdoor air, so it is best treated as a supplement for rooms that genuinely cannot be ventilated rather than as a substitute.

How long should I open a window for?

Opening windows fully for a few minutes is generally more effective than leaving one slightly ajar for hours, which also lets the room warm back up quickly afterwards.

Do carbon dioxide monitors detect COVID-19?

They do not detect the virus at all. They indicate how much exhaled air is building up in a room, which is a rough proxy for how well that space is being ventilated.

Does a ceiling fan help reduce transmission?

On its own it mainly moves the same air around the room, though it can be useful when combined with an open window to help push indoor air outside.

Should I run an air cleaner all the time?

Running it during the hours a room is occupied is usually the sensible approach, provided the unit is rated for the size of the room and the filter is replaced on schedule.

Does ventilation help with anything other than COVID-19?

Yes, the same principle applies to other respiratory infections including flu and colds, which is part of why ventilation guidance has outlasted most other pandemic measures.