The Covid-19 virus can spread from an infected person’s mouth or nose in small liquid particles when they cough, sneeze, speak, sing or breathe heavily. These liquid particles are different sizes, ranging from larger ‘respiratory droplets’ to smaller ‘aerosols’. The virus can also be spread through contact with contaminated surfaces, hence the guidance to socially distance and wash hands regularly.

Larger droplets can land on other people or on surfaces they touch. Spreading the virus through droplets is most likely to happen when you are less than 2m apart. Aerosols are much smaller than droplets and can travel further and remain suspended in the air for longer, particularly in poorly ventilated indoor spaces.

Good ventilation is therefore an important component of an employer’s overall strategy to reduce Covid-19 in the workplace. In particular ensuring multi-occupant workplaces where individuals are in the same room/space together for an extended period of time are well ventilated to help prevent the build-up of the virus and reduce the risk of transmission.

The Workplace (Health, Safety and Welfare) Regulations (Northern Ireland) 1993 require all employers to ensure that every enclosed workplace is ventilated by a sufficient quantity of fresh or purified air and this has not changed.

If the ventilation strategy is achieving the standards set then only minor changes may be needed. Employers should review the ventilation arrangements within their premises and update their workplace Covid-19 risk assessments.

Further information:

What is the difference between droplets and aerosols?

Droplets are larger respiratory particles with the majority between 10 -100 microns diameter but they can be up to 1500 microns in diameter.  These normally settle out of the air in less than 5 minutes and within 2 meters of the source/infected person.  

Aerosols are small respiratory particles less than 10 microns in diameter and can remain airborne for longer periods and potentially inhaled.  The greatest risk of inhalation is within 2 meters of an infected person but aerosol can remain in the air at distances more than 2 meters and in excess of 5 minutes and potentially hours where there is poor ventilation.

Face coverings are mainly intended to protect others and against droplet transmission. When used correctly, they cover the nose and mouth, which are the main sources of transmitting coronavirus (Covid-19), confining/containing droplets and larger particles and helping prevent them from entering the general environment. There are some circumstances when wearing a face covering is required as a precautionary measure. You can find out when workers need to wear a face covering on GOV.UK. Face coverings alone may not adequately reduce the risk of airborne transmission and should not be used as a replacement to good ventilation.

Further information:

Why is ventilation important?

Even with other controls in place (including; social distancing, frequent hand washing, increased cleaning regimes, use of face coverings/transparent barriers/shields etc.), employers should still consider the risk from airborne transmission of the Covid-19 virus from aerosols in poorly ventilated indoor spaces, particularly if individuals are in the same room together for an extended period of time. Ventilation should therefore be considered as an important part of the hierarchy or risk controls

Increasing the occupancy of a space also increases the probability of airborne transmission. Ensuring a space has good ventilation and an adequate supply of fresh air can therefore help reduce the risk of spreading Coronavirus.

Is there a legal requirement to ensure good ventilation?

The Workplace (Health, Safety and Welfare) Regulations (Northern Ireland) 1993 require all employers to ensure that every enclosed workplace is ventilated by a sufficient quantity of fresh or purified air and this has not changed.

Which areas are a priority to ensure good ventilation?

Mitigation of the risk from multi-occupant spaces where individuals are in the same room/space together for an extended period of time with low ventilation rates should be prioritised to improve the supply of fresh air over adequately ventilated areas. 

Other poorly ventilated occupied spaces and environments that may pose an enhanced risk of transmission e.g. due to low temperature and low humidity (chilled food processing, cold stores etc.) or higher levels of aerosol generation (Singing, loud speech, aerobic activity/dancing, dental and medical procedures etc.) may also require improvements to ventilation.

Improving ventilation does not remove the need for the employer to implement other controls to reduce the risk of transmission of Covid-19.

What affect does the work area have on the spread of Covid-19?

The size of the space being occupied is an important factor regarding the spread of Covid-19.  In a large space with low occupancy, such as a warehouse, even with a low ventilation rate it will take a long time for virus concentration to build up.  Short duration exposures could be of low risk but occupancy will affect the virus build up.

Each scenario would have to be considered on a case by case basis to determine the level of ventilation needed.  Unfortunately one size doesn’t fit all. An example would be comparing an open plan air conditioned office/workplace with an open large manufacturing workshop or warehouse clearly will have distinct requirements.

What is adequate ventilation?

Ventilation should be an integral part of an organisations Covid-19 risk management strategy. This should include how a space is ventilated and the strategy to ensure the ventilation is adequate.

Ventilation is one of the most important factors in mitigating the risk of aerosol transmission beyond 2 meters. Poor ventilation can increase the risks of aerosol transmission beyond 2 meters and build-up of infective aerosol in the occupied space.

Aerosol transmission will be governed by the viral emission rate, duration, environmental conditions, number of occupants and ventilation.

A poorly ventilated area can be described as one that has movement of air that is below 5 Ltrs/sec/person or above 1500ppm of Carbon Dioxide (CO2).

An adequately ventilated area can be described as having a movement of air that has 8-10 Ltrs/sec/person or below 800ppm of CO2. In communal areas such as offices around 1000ppm of CO2 is widely regarded as an indicator of sufficient per person ventilation rate.

How do I know if my workplace is poorly ventilated?

Every work place is different, due to the industry it is in, the location, construction and age of building, level and type of occupancy, type of ventilation etc. therefore determining whether a work place is well ventilated or not is not straight forward and in many environments requires engineering expertise.

Measurements of elevated carbon dioxide levels in indoor air can be an effective method of identifying poor ventilation in multi-occupant spaces (not suitable for low occupancy or large volume spaces or industrial processes that release or generate carbon dioxide which could affect readings should be taken into account).

The Scientific Advisory Group for Emergencies (SAGE) have recommended that multi-occupant spaces that are used regularly and have above 1500ppm CO2 are prioritised for improving ventilation.

The amount of fresh air supplied per person and the number of times the air is replaced within a space is also an indicator of how well it is ventilated. If it is not meeting the minimum 5-8 Ltrs/sec/person as detailed in the guidance to the Workplace (Health, Safety and Welfare) Regulations (NI) 1993 or the recommended design requirements of the Chartered Institution of Building Services Engineers (CIBSE) guidance B2 then it could be considered poorly ventilated.

Access to these documents are available within the useful links section at the end of this page.

How can ventilation be improved in the workplace?

If the ventilation strategy for the workplace is achieving the standards set out in the guidance to the Workplace (Health, Safety and Welfare) Regulations (NI) 1993 and CIBSE building guidance, B2 Ventilation and Ductwork, then only minor changes may be needed. 

Measures to improve ventilation should not be taken in isolation and should be part of a combined approach to reduce the overall risk of transmission.  It will also be dependent on the particular setting and activity taking place. There is no one solution that is suitable for all scenarios. Brief points on how to improve ventilation are set out below and should be considered alongside the guidance given by CIBSE (2020) and the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA, 2020).

Air change rates for replenishing internal air from an external source will assist in removing aerosol. Internal air circulating fans, recycling air conditioning units will not add to the air change rate and although resulting in a more comfortable perceived atmosphere, may well not decrease the risk of infection or concentration of aerosol. Recirculation or transfer of air from one room to another should be avoided unless this is the only way of providing adequate ventilation rates to all occupied rooms. This is a complex area and each case will need to be assessed by competent ventilation engineer.

1. Natural ventilation  
Natural ventilation consists of opening windows and doors.

Changes to improve ventilation can be achieved by:

  • partially opening doors and windows to provide ventilation while reducing draughts;
  • opening high level windows in preference to low level to reduce draughts; and
  • purging spaces by opening windows, vents and external doors (e.g. at suitable intervals if a space is occupied for long periods at a time).

Options to improve thermal comfort include;

  • adjusting indoor heating systems to compensate for cold air flow from outside (e.g. higher system settings, increased duration);
  • work locations could be relocated to avoid drafts and maintain social distancing; and
  • encourage staff to dress appropriately for the indoor temperature.

A strategy of short duration purging at regular intervals can be effective in controlling air quality and thermal comfort this will have to be determined on a case by case basis depending on building design, size, age etc.

Tradesmen who are working in premises (including domestic premises) should aim to ensure adequate ventilation is available by opening windows and purging work areas (rooms).

2.   Mechanical ventilation (HVAC)

Mechanical ventilation is a method of forced or induced ventilation by using mechanical air handling systems, commonly called HVAC systems. Mechanical ventilation is more controllable than natural ventilation.

  • Increase the air supply and exhaust ventilation, supplying as much outside air as is reasonably possible.
  • Recirculation or transfer of air from one room to another should be avoided unless this is the only way of providing adequate ventilation rates to all occupied rooms.
  • Extend operating times of the ventilation system.
  • Change CO2 set point to a lower value to ensure systems do not automatically adjust ventilation levels due to differing occupancy levels.

3. Air conditioning units (with HEPA filter installed)

Air conditioners with HEPA filters installed often use a fan to distribute the conditioned air to an enclosed space such as a building.  HEPA stands for ‘High-Efficiency Particulate Arresting’.  HEPA filters including air purification technology inbuilt to air conditioners are used to improve air filtration and quality.

  • Recirculation units for heating and cooling that do not draw in a supply of fresh air can remain in operation provided there is a supply of outdoor air, for example windows and doors left open.
  • Recirculation units (including air conditioning) can mask poor ventilation as they just make an area more comfortable.

I’m using natural ventilation but the workplace is cold.  What do I do?

The Workplace (Health, Safety and Welfare) Regulations (NI) 1993 require the temperature in all workplaces inside buildings to be reasonable. There is a balance to be struck between an appropriate supply of fresh air to assist with minimising the risk of virus transmission and the need to maintain indoor temperatures for reasons including occupant comfort, health and wellbeing.

Scientific and public health advice is that measures to introduce fresh air can have a beneficial impact on the suppression of virus transmission.  Employers should involve employees in the development of the ventilation strategy and communicate it to the workforce.  Pragmatic approaches which recognise the importance of user comfort will help with overall behavioural adherence to guidance in relation to ventilation.

In naturally ventilated buildings, those that do not have a heating and ventilation system, strategies such as intermittent airing and partial window opening to complement background ventilation may enable sufficient ventilation without impacting too much on thermal comfort.

Further information:

Will environmental factors such as sunlight (UV) effect Covid-19 aerosols?

Covid-19 aerosols have been shown to be stable in air for several hours with humidity only having minor effect.

While normal summer equinox UV levels (London equivalent) estimated to result in a 90% reduction in viral infectivity in 30 mins, the time taken for similar loss of infectivity at the vernal, spring and autumn equinox are estimated to be 77, 173 and >300 minutes (5 hours).

Therefore ventilation is even more important in the absence of natural sunlight as virus will survive longer and this should be considered in the workplaces Covid-19 risk assessment.

Therefore outdoor virus contaminated surfaces likely to retain infectivity far longer in winter compared to summer.

*Information obtained from: Seasonality and its impact on COVID 19. (Joint NERVTAG/EMG Working Group. Kath O’Reilly, John Edmunds, Allan Bennet, Jonathan Reid, Peter Horby, Catherine Noakes). 21 October 2020


This is a complex area and one solution will not solve all situations.  Carbon dioxide (CO2) levels can be used as well as air supply (which is measured in Ltrs/sec/person) to assess and monitor the ventilation in a workplace. 

The risk of transmission of COVID-19 from aerosols and the ventilation strategy needed to reduce the risk, should be considered as part of workplace’s overall risk assessment for COVID-19.

High risk areas, such as multi-occupant spaces that are used regularly and are poorly ventilated should be identified and prioritised for improvement. 

Employers should endeavour to minimise the transmission of aerosol/droplets in the workplace and employees cooperate with the systems of work to ensure so far as is reasonably practicable, that airborne infective aerosols are minimised/eliminated. Specialist competent help may be needed to risk assess your workplace to achieve the required objectives.

A new Government video to show the importance of ventilation in reducing Covid-19 risks is available via the following link:

Useful Links

A number of government and professional bodies have released detailed guidance on the importance of good ventilation and ways to improve ventilation within buildings: