Ventilation Guidance

What is ventilation and why does it matter?

Ventilation is the process of refreshing indoor air by allowing air to flow into a space whilst letting air (which may be stale) out. Ventilation helps us maintain good air quality in indoor spaces (see our webpages defining air quality and some key indoor air quality metrics), by diluting and removing pollutants from the air indoors. Ventilation is therefore important for promoting good health, including reducing the spread of respiratory infections such as COVID-19 or the flu, and it has been linked to better sleep and better concentration for learning. For further information, check out our Air Pollution and Health page.

Summary of ventilation recommendations for schools

• Many of the spaces within our schools, including most classrooms, are naturally ventilated by airflows through open windows and doors (see below)
• In naturally ventilated rooms, open windows, doors and other vents
• To maximise natural ventilation:
  • Windows/doors may need to be opened wider at both at high- and low-levels within the room, and/or
  • Windows/doors may need to be opened on opposing sides of the room.
• During warm weather ventilation can typically be maximised without compromising thermal comfort, and will not affect heating costs or energy consumption.
• During colder periods, smaller window openings are sometimes sufficient to achieve adequate ventilation. In classrooms where it is an option, open high-level windows first to maintain thermal comfort.
• CO₂ levels provide an indication of the amount of ventilation relative to the occupancy within teaching spaces. So CO₂ monitors can be a good tool to decide when more or less ventilation is required.
  • Actions to improve ventilation should be taken when CO₂ levels are consistently above 1,500ppm, you can use the table on our guidance on monitor readings page to help you manage your ventilation.
  • If CO₂ levels consistently remain above 1,500ppm, and your actions do not improve this, then please seek support from your Senior Leadership Team.
• Mechanical ventilation systems should be maintained to manufacturer standards by trained HVAC personnel.

Types of ventilation

Most UK classrooms are naturally ventilated. Some are mechanically ventilated, and some are mixed-mode or hybrid, which means they have a mix of both natural and mechanical ventilation.

Natural ventilation relies on natural processes, such as wind and temperature differences, to drive air in and around buildings through openings such as windows, doors, chimneys, wind catchers, among others. This works because of laws of physics. Where there is a pressure difference between two bodies of air, the air will move from high pressure to low pressure. Wind blowing outside a window, and temperature differences between indoors and outdoors, create pressure differences and so can drive ventilation. Inside our schools, warm air rises and escapes through the upper portions of windows and any high-level vents, as this air escapes, air from outside is pulled in through the lower portions of windows and vents.

If you have openable windows or a door to outdoors then your classroom has at least some natural ventilation. Note that fire doors should not be propped open and, therefore, fire doors cannot be regarded as providing ventilation.

Mechanical ventilation systems mainly use fans to move fresh air in and stale air out of spaces. The air moves along ducts to vents (often fitted with grills) on the ceilings, walls or floor. Grills are also found in extract ventilation (often used in toilets and kitchens), but this only takes stale air out of rooms, and does not actively manage the incoming air supply, so is not considered mechanical ventilation.

It can be hard to tell if you have mechanical ventilation as usually ducts are hidden behind walls or in ceilings, so look for covers over vents. Check with your Senior Leadership Team or site management staff if you are unsure.

For more information on types of ventilation, see the National Engineering Policy Centre pages.

Recommendations to improve ventilation

• Improving ventilation in naturally ventilated rooms

  In warmer weather, the best thing to do is open windows. In colder weather, there are concerns about heating costs, energy consumption, and cold draughts. The CO₂ and temperature readings on your SAMHE monitor can help to manage the balance between thermal comfort, the heating cost of energy consumption and air quality. You may want to look at readings before pupils arrive and over breaks, to see if ventilation needs to be improved. In colder months, you may wish to periodically ventilate during breaks, to maintain a comfortable temperature. Here are some recommendations for different natural ventilation systems:

  • All windows on one side, and at similar heights. In warm months, all windows might need to be opened wide to increase ventilation to keep indoors cool and improve indoor air quality. In cooler months, cooler air will tend to flow through the lower portion of the windows, or particular windows depending on the wind. You can try opening different windows, to different extents, to try to avoid the cold outside air directly hitting people. If CO₂ levels need decreasing then try opening windows wider, but do so gradually to avoid indoors becoming too cold.
  • High and low windows/openings. In warm months, open low and high windows to increase airflow between them, cooling the air and improving indoor air quality. In cooler months, try opening high windows first to mix stale air with fresh air without the cold outside air directly hitting people. If CO₂ levels need decreasing further then low level windows should be opened.
  • Windows / doors on different sides of the room. Opening windows/doors on opposite sides of the room makes air move from one side to the other, ventilating the room. In colder months, opening all windows by a small amount will provide better airflow and be warmer for occupants than opening a single window widely.
  • Sash windows (those with panels that move vertically). In colder months, open the high-level sash first to keep the room a comfortable temperature, if CO₂ levels do not drop then the low level sash windows too. In summer months, open both top and bottom levels.

• Improving ventilation in mechanically ventilated rooms

  Mechanical ventilation systems should be properly maintained by trained staff following the manufacturer's instructions, including regularly cleaning ducts, changing filters, etc.

  Some systems are controlled by temperature and/or CO₂ sensors and ensuring that adequate set points are given is important to provide good ventilation. It is important to make sure that the mechanical ventilation systems are bringing in an appropriate amount of outdoor air. Partial air recirculation can be appropriate, particularly when combined with filtration (for example HEPA filters). These filters do not filter out CO₂ so if you have a CO₂ monitor then the concentrations that you see might be higher than those typically recommended and careful interpretation might be required.

Using CO₂ monitors to indicate ventilation levels

CO₂ monitors can be used to indicate the per person ventilation in a room, particularly in naturally ventilated classrooms. They are less useful when the number of occupants are low, in very large rooms, or in the presence of filters.

The SAMHE monitors can be used to help decide whether ventilation needs to be improved. Our guidance on monitor readings page gives an overview of how your monitor will look at different CO₂ concentrations and indicates any actions that could be taken.

Following guidance from the UK's Scientific Advisory Group for Emergencies (SAGE), we take 800 ppm and below to indicate good ventilation. Above this, we recommend action should be taken to improve ventilation. For more information, see the DfE Guidance for naturally ventilated classrooms in BB101.

Regularly monitoring CO₂ should allow you to spot any changes, take action and see the effects of those actions. Please note that even at high concentrations (over 1,500ppm) CO₂ levels are not directly dangerous to our health and there is no need to evacuate classrooms. Carbon dioxide, CO₂, is not a poisonous gas (unlike carbon monoxide, CO).

In some cases, high CO₂ readings can be due to a monitor fault, please see our Monitor placement and accuracy guide.

If CO₂ levels in your classroom consistently rise above 1,500ppm, and any actions you take do not improve ventilation, please speak to your Senior Leadership Team. In such circumstances, long-term solutions to improve the classroom ventilation provision may need to be identified and implemented; the addition of stand-alone HEPA filter units may be a viable solution in the interim.

Ventilation and the COVID-19 pandemic

Throughout the pandemic, there has been much focus on the link between reducing the spread of infection, and increasing ventilation. Respiratory infections such as COVID-19, or the flu, can be transmitted through the air. For more information see our Air pollution and health page.