What humidity should an office be in the UK?

What does UK law say about office humidity?

There is no legal humidity range for UK workplaces. The closest the law comes is in the Health and Safety (Display Screen Equipment) Regulations 1992, which apply to staff who use screens as a significant part of their work, so most office-based staff. Paragraph 3(g) of the Schedule says: “An adequate level of humidity shall be established and maintained.”

HSE’s guidance on those regulations (L26) adds that electronic equipment can be a source of dry heat at the workstation, and that ventilation and humidity “should be maintained at levels which prevent discomfort and problems of sore eyes.”

The Workplace Regulations Approved Code of Practice (L24) mentions humidity only in passing. It lists relative humidity as one of the factors, with air movement and clothing, that affect whether a temperature feels reasonable. It also asks that air which is hot or humid because of processes or equipment is replaced at a reasonable rate.

So the legal test is “adequate” and “reasonable”. The working figure comes from HSE’s thermal comfort guidance.

Where does the 40 to 70 % figure come from?

HSE’s page on the six basic factors of thermal comfort puts the comfortable band at 40 to 70 % relative humidity. It also notes that in workplaces without air conditioning, relative humidity may be higher than 70 %.

That is a comfort band, and HSE does not present it as a legal limit or a health threshold. Inside it, humidity is unlikely to be the reason people feel uncomfortable.

What is relative humidity?

Relative humidity is a percentage. HSE defines it as the ratio between the actual amount of water vapour in the air and the maximum amount the air can hold at that air temperature.

The important part is “at that air temperature”. Warm air can hold more water vapour than cold air. So the same amount of moisture gives a lower relative humidity in a warm room and a higher one in a cold room. This is why a temperature reading and a humidity reading are most useful side by side.

Why is office air so dry in winter?

Cold outdoor air holds little water vapour. When it comes inside and the heating warms it up, the amount of moisture stays roughly the same but the air can now hold much more. Its relative humidity drops.

IT kit adds to it. L26 notes that electronic equipment can be a source of dry heat at the workstation.

What happens when office air is too dry?

The effect HSE names is discomfort and sore eyes, which is the problem L26 tells employers to prevent through ventilation and humidity. People who spend the day at screens are the ones most likely to notice.

Complaints about dry eyes in winter are reason enough to look at the readings.

What happens when it is too humid?

High humidity has two separate effects.

The first is comfort. HSE explains that a lot of vapour in the air prevents the evaporation of sweat from the skin. In hot conditions it singles out humidity above 80 % as the point where less sweat evaporates. In a UK office this mostly shows up in summer, in rooms without cooling, as a stuffy, sticky feel.

The second is the building. Government guidance on damp and mould says condensation is one of the most common causes of damp and mould. It happens when moisture generated inside cools and condenses onto colder parts of the building, such as window frames, corners and low points on walls behind furniture. That guidance is written for homes, but the physics is the same in an office, especially in older buildings and in kitchens, showers and basement rooms.

The same guidance says damp and mould “primarily affect the airways and lungs”, with effects including cough, wheeze, shortness of breath and worsening asthma. It lists consistently high relative humidity, assessed with a moisture meter or environmental monitors, as one of the things to check for. If one room keeps reading high, find out why before it shows on the walls. Our guide to monitoring old buildings without rewiring covers why moisture matters so much in traditional buildings.

Where should you measure humidity in an office?

Measure the air people actually work in. The same rules apply as for temperature:

• Roughly at seated head height, in the main working area.
• Away from radiators and heaters, which push relative humidity down locally.
• Away from windows and external walls, where the air is colder and relative humidity reads higher.
• Away from kitchens and washrooms, unless you are measuring those rooms on purpose.
• One per zone on a large open floor, and one in any meeting room that gets crowded.

Then measure the rooms where problems start: basements, rooms on north-facing walls, kitchens and anywhere with a history of condensation. A single reading tells you very little. A few weeks of readings show you which rooms drift, at what time of day, and whether it lines up with the heating schedule.

Comms rooms and server cupboards are a different case with their own guidance. See what temperature a server or comms room should be, which covers humidity for IT equipment too.

What should you do if humidity is out of range?

Start with the cause, then pick the fix.

If it is too high:

• Improve ventilation. L24 asks for air made humid by processes or equipment to be replaced at a reasonable rate.
• Use extract fans in kitchens and washrooms, and keep them running long enough to clear the moisture.
• Look for cold surfaces where condensation forms, and for leaks or rising damp, which ventilation alone will not fix.
• A dehumidifier can help in a problem room while the cause is dealt with.

If it is too low:

• Check the ventilation and heating settings first. Over-ventilating cold air and running the heating hard is the usual cause.
• Keep people away from direct heat sources where you can.
• If you add humidifiers, maintain them. HSE lists humidifiers and air washers among the systems that could be a source of legionella growth. It says such systems should be kept clean and monitored, that a risk assessment is needed, and that there is a duty to keep records of maintenance and monitoring. People catch Legionnaires’ disease by inhaling small droplets of water, which is exactly what some humidifiers produce.

Set an alert band around 40 to 70 %, with a delay so a busy meeting or a boiling kettle does not set it off. Then adjust once you know how your rooms behave.