Ventilation (COVID-19)

Good ventilation is one of the key controls to prevent the spread of COVID-19. Used along with social distancing, good hygiene and the use of face coverings where appropriate, good ventilation can significantly reduce the risks to individuals and the business.


The transmission of COVID-19 occurs mainly through the spread of respiratory droplets, from coughing, sneezing, talking etc., and through contact with contaminated surfaces. Larger droplets will generally fall out of the airstream within a short distance (depending on airflow speed and direction), hence the guidance to socially distance and wash hands regularly.

However, evidence which is acknowledged by the World Health Organisation (WHO) and Public Health England (PHE) is that the COVID-19 virus is spread through smaller droplets in the air called aerosols. Aerosols are much smaller than droplets and can travel further and remain suspended in the air for longer, particularly in poorly ventilated indoor spaces.

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 five minutes and within two 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 two meters of an infected person but aerosol can remain in the air at distances more than two meters and in excess of five minutes and potentially hours where there is poor ventilation.

Why is Ventilation Important?

Airborne transmission of the COVID-19 virus from aerosols can occur in poorly ventilated indoor spaces, particularly if individuals are in the same room together for an extended period of time. 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.

The highest risks of transmission are associated with poorly ventilated, crowded indoor settings and not wearing face coverings.

Good ventilation is also an important part of the hierarchy of risk controls. Other factors such as frequent hand washing/sanitising, restricting activities, reducing activity time, use of face coverings/transparent barriers/shields and social distancing should also be used to minimise cross infection.

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 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 for example 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 such as, home working where possible, social distancing, increased cleaning and decontamination etc.

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 two meters. Poor ventilation can increase the risks of aerosol transmission beyond two 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 area with adequate ventilation 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.

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

The size of the space being occupied is an important factor. In a large space such as a large 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 and occupancy will affect the virus build up.

Each case would have to be considered on a case by case basis to determine the level of ventilation needed, 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.

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 low occupancy or large volume spaces).

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 / second / person as detailed in the guidance to the Workplace (Health, Safety and Welfare) Regulations (NI) 1993 or the recommended design requirements of CIBSE guidance B2 then it could be considered poorly ventilated.

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.

Naturally ventilated  

Natural ventilation consists of opening windows and doors.

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.

Air change rate for replenishing internal air from an external source will assist in removing aerosol but 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. This is a complex area and each case will need to be assessed by a competent ventilation engineer.

Mechanical ventilated (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.

Tradesmen working in premises including domestic premises should ensure adequate ventilation is available by opening windows and purging rooms / work areas would be good practice.

Air conditioning units with HEPA filter installed

Air conditioners 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.

The workplace is cold - Do windows have to be open?

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.

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
  • 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
  • encourage staff to dress appropriately for the indoor temperature

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.

During summer months, sunlight (UV) results in decay/survival time for the airborne virus to be around eight minutes. During spring/autumn this increases to approximately 19 minutes due to reduced levels of UV.  However in the absence of UV light (some factory settings) survival time can increase to around 286 minutes i.e. approaching five hours. This is not to be confused with the survival time of Coronavirus on solid surfaces which can be up to 72 hours depending on the host material.


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. Risk areas should be identified wherever possible with the higher risk areas being addressed first. 

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

SAGE (Scientific Advisory Group for Emergencies) Environmental and Modelling Group - Role of Ventilation in Controlling SARS-CoV-2 Transmission:

Heating, ventilation and air-conditioning systems in the context of COVID-19: 

How to operate HVAC and other building service systems to prevent the spread of the coronavirus (SARS-CoV-2) disease (COVID-19) in workplaces:

Coronavirus, SARS-COV-2, COVID-19 and HVAC Systems and CIBSE COVID-19 Ventilation Guidance: