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The priority for your risk assessment is to identify areas of your workplace that are usually occupied and poorly ventilated.

There are some simple ways to identify poorly ventilated areas:

• Look for areas where people work and where there is no mechanical ventilation or natural ventilation such as open windows, doors, or vents.
• Check that mechanical systems provide outdoor air, temperature control, or both. If a system only recirculates air and has no outdoor air supply, the area is likely to be poorly ventilated
• Identify areas that feel stuffy or smell bad

Using carbon dioxide (CO2) monitors

People exhale carbon dioxide (CO2) when they breathe out. If there is a build-up of CO2 in an area it can indicate that ventilation needs improving.

Checking levels of carbon dioxide (CO2) using a monitor can help you identify poorly ventilated areas.

Types of CO2 monitor to use

There are many different types of CO2 monitors available. The most appropriate portable devices to use in the workplace are non-dispersive infrared (NDIR) CO2 monitors.

How to use a CO2 monitor

CO2 levels vary within an indoor space. It’s best to place CO2 monitors at head height and away from windows, doors, or air supply openings.

Monitors should also be positioned at least 50cm away from people as their exhaled breath contains CO2. If your monitors are too close they may give a misleadingly high reading.

Measurements within a space can vary during the day due to changes in the numbers of occupants, activities, or ventilation rates. Doors and windows being open or closed can also have an effect.

The amount of CO2 in the air is measured in parts per million (ppm). If your measurements in an occupied space seem very low (around 400ppm) or very high (over 1500ppm), it’s possible your monitor is in the wrong location and you should move it to another location in the space to get a more accurate reading.

Instantaneous or ‘snapshot’ CO2 readings can be misleading, so you should take several measurements throughout the day frequently enough to represent changes in use of the room or space. Then calculate an average value for the occupied period.

You may need to repeat monitoring at different times of the year as outdoor temperatures change and this will affect worker behaviour relating to opening windows and doors when your space relies on natural ventilation.

Your readings will help you decide if a space is adequately ventilated.

How to get the most accurate readings

• Check your monitor is calibrated before making CO2 measurements. Follow the manufacturer’s instructions, including the appropriate warm-up time for the device to stabilise
• Know how to use your portable monitor correctly, including the time needed to provide a reading
• Take multiple measurements in occupied areas to identify a suitable sampling location to give a representative measurement for the space. In larger spaces it is likely that more than one sampling location will be required
• Take measurements at key times throughout the working day and for a minimum of one full working day to ensure your readings represent normal use and occupancy
• Record CO2 readings, number of occupants, the type of ventilation you’re using at the time and the date. These numbers will help you use the CO2 records to decide if an area is poorly ventilated

How the measurements can help you take action

CO2 measurements should be used as a broad guide to ventilation within a space rather than treating them as ’safe thresholds’.

Outdoor CO2 levels are around 400ppm.

Indoors, a consistent low CO2 reading is likely to indicate a better ventilated space with a higher air quality:

Source: BS EN 13779

An average of 1500ppm CO2 concentration over the occupied period in a space is an indicator of poor ventilation. You should take action to improve ventilation where CO2 readings are consistently higher than 1500ppm.

However, where there is continuous talking or singing, or high levels of physical activity (such as dancing, playing sport or exercising), providing ventilation sufficient to keep CO2 levels below 800ppm is recommended.

Where CO2 monitors will be less effective

CO2 monitors are not suitable for use in areas that rely on air cleaning units because these remove contaminants (such as coronavirus) from the air but do not remove CO2.

In large, open spaces and spaces with higher ceilings, such as food production halls or warehouses, you can’t be sure the air is fully mixed and CO2 monitors may be less representative.

Monitors are of limited use in less populated areas. These include fitting rooms or large offices with one or two occupants.

The Scientific Advisory Group for Emergencies (SAGE) has published a paper on the use of CO2 monitoring. The table below gives examples of spaces where monitors may be useful.

Although this table gives some examples, every space is different, and you need to consider whether a CO2 monitor will be appropriate for you.

Suitability of CO2 monitoring in different types of space

There are several factors to consider when deciding on the ventilation needed in your work areas.

You must make sure there is an adequate supply of fresh air in any workspace. You can do this by using:

• natural ventilation - fresh air comes in through open windows, doors or air vents. This is also known as ‘passive air flow’
• mechanical ventilation - fans and ducts bring in fresh air from outside

There may already be different types of ventilation around your workplace. It may help to make a list of areas in your workplace and how they are ventilated. Floor or design plans may help with this.

Alternatively, you could walk around the building and make a note of each area and how it is ventilated. Remember to include changing rooms and areas used for breaks, such as canteens. If you can’t tell easily how an area is ventilated, it may be because it is poorly ventilated.

How many people use or occupy the area?

The more people who use or occupy an area, the greater the risk that an infected person is there, increasing possible exposure to aerosol transmission. The risk increases if an area is poorly ventilated and occupied by more than one person.

Consider how many people use or occupy an area at any one time. Is there a set number of people each day or do numbers fluctuate?

How large is the area?

The larger the area, the lower the risk. This is because larger areas:

• have more air to help dilute the virus
• tend to be designed with ventilation rates in mind
• take longer for aerosols to build up in them

What tasks or activities take place in the area?

Activities that make you breathe deeper, for example physical exertion or shouting, will increase:

• generation of aerosols
• risk of transmission

Activities like these increase transmission risk even where there’s adequate ventilation. If possible, avoid or redesign these activities to reduce the risk. This could include moving some activities outside or working alone where possible.

Are there any features in the workplace that affect ventilation?

You may have large machinery, equipment or other features that could prevent air from circulating in your premises. These features could include things like pillars or posts. Large machinery or physical features could make the air stagnant. Consider how to improve airflow in the area.

Do you use desk or ceiling fans?

You should not use desk or ceiling fans in poorly ventilated areas.

Does your workplace use local exhaust ventilation?

You may use local exhaust ventilation (LEV) to control risks from other workplace hazards such as dust or welding fumes. If an LEV system discharges the air outside, it will improve ventilation in the area.

Is there a complex ventilation system?

Workplaces that may have complex ventilation systems include:

• some old buildings
• buildings with multiple floors and rooms using different ventilation systems
• systems designed for product manufacturing as these can include additional recirculation

If your workplace has a complex ventilation system, there is .

You may need a ventilation engineer to provide expert advice on the best system for your workplace.

How will you tell your employees about the outcome of your assessment?

You should tell your workers about the outcome of the risk assessment.

You can improve natural ventilation by fully or partly opening windows, air vents and doors. Don’t prop fire doors open.

Buildings are usually designed to provide adequate ventilation. You should be able to open any windows or vents that let in fresh air. If they cannot be opened, ventilation in that area will be less effective.

If you identify an area that needs improvement, you should decide if it’s safe for people to use that area before you make any changes.

Don’t close doors or windows completely when people are in a naturally ventilated area. This can result in very low levels of ventilation.

Airbricks and ventilation grids need to be kept clean, so that the air supply is not obstructed, and where possible open any trickle vents in your workplace.

Cooler, windier weather increases natural ventilation through openings. This means you don’t need to open windows and doors so wide.

We have more advice on balancing ventilation with keeping workplace temperatures comfortable.

The Health & Safety Executive in Great Britain have produced .

Purging (airing rooms)

Airing rooms as frequently as you can improves ventilation. Opening all the doors and windows maximises ventilation in a room. It may be better to do this when the room is unoccupied. The use of a CO2 monitor will help you judge how long it takes to purge the air in a room.

Talking to your workers about improving ventilation

Making sure that an area has enough fresh air relies on your workers playing their part. You should explain the importance of adequate ventilation to your workers.

Mechanical ventilation brings fresh air into a building from outside.

You should speak to the people who manage the day-to-day operations of your workplace’s mechanical ventilation systems to:

• understand how they operate
• make sure they’re supplying fresh air into an area and how much
• make sure they’re maintained in line with manufacturers’ instructions

Don’t lower mechanical ventilation rates if the number of people in an area reduces temporarily.

You should base ventilation rates on the maximum ‘normal’ occupancy of an area.

Maximising fresh air

Mechanical systems will provide adequate ventilation if they are set to maximise fresh air and minimise recirculation.

If your system draws in fresh air, it can continue to operate. You need to know how much fresh air it draws in and if this provides adequate ventilation. You may need to increase the rate or supplement it with natural ventilation (for example, by opening doors, windows or air vents) where possible.

You could also consider extending the operating times of mechanical ventilation systems to before and after people use work areas.

Recirculating air

It’s better not to recirculate air from one space to another. Recirculation units for heating and cooling that do not draw in a supply of fresh air can remain in operation as long as there is a supply of outdoor air. This could mean leaving windows and doors open.

Recirculation units (including air conditioning) can mask poor ventilation as they only make an area feel more comfortable.

Find out more

The Health & Safety Executive in Great Britain have produced examples of .

Providing adequate ventilation does not mean people have to work in an uncomfortably chilly or cold workplace.

There are simple steps you can take to make sure your workplace is adequately ventilated without being too cold:

• Partially opening windows and doors can still provide acceptable ventilation while keeping workplace temperatures comfortable.
• Opening higher-level windows will probably create fewer draughts.
• In occupied rooms relying on natural ventilation, air the space by opening windows and doors as fully as possible to regularly provide additional fresh air.
• This can be done while people leave the room for a break. For example, 10 minutes an hour can help reduce the risk from virus in the air, depending on the size of the room.
• If the area is cold, relax dress codes so people can wear extra layers and warmer clothing.
• You could set the heating to maintain a comfortable temperature even when windows and doors are open.

Consider providing additional sources of heating if required. Only use fan convector heaters if the area is well ventilated.

Employers have a legal duty to ensure that working environments are a 'reasonable temperature'. Find out more about safe working temperatures. 

You can use local air cleaning and filtration units to reduce airborne transmission of aerosols where it is not possible to maintain adequate ventilation.

These units are not a substitute for ventilation. You should prioritise any areas identified as poorly ventilated for improvement in other ways before you think about using an air cleaning device.

If you decide to use an air cleaning unit, the most suitable types to use are:

• high-efficiency filters
• ultraviolet-based devices

Any unit should be appropriate for the size of the area it’s used in to ensure it works in the way it’s intended to.

Carbon dioxide (CO2) monitors are not suitable for use in areas that rely on air cleaning units. This is because filtration units remove contaminants (such as coronavirus) from the air but do not remove CO2.

Make sure workers switch on ventilation systems while they’re using work vehicles. It is important to remember that they should be set to draw in fresh air and not to recirculate cabin air. Encourage your employees to keep vehicle windows open. If it’s cold they can leave the heating on to keep the vehicle comfortable.

If it’s safe to do so, opening doors of vehicles at stops or between different passengers will help to change the air quickly. Keeping windows open when the vehicle is moving or opening vehicle windows fully for a few minutes when stopped can also help clear the air.

The Department for 91Ï㽶»ÆÉ«ÊÓƵ has guidance on .

The Department for Infrastructure has guidance for .

You should also make sure any control measures you identify by your risk assessment take account of the most up to date public health regulations and guidance.