Last reviewed 5 July 2022

Most people instinctively fling open a window to improve indoor air quality. That will reduce toxin levels for a while say experts, but close them tightly again to avoid pollution from roads, railways and industrial sites. Keeping the indoor atmosphere safe is now complex. Jon Herbert reports.

Once upon a time, it was reasonable to assume that letting “fresh” air into homes and workplaces was the best thing to do when it got a bit stuffy indoors. Today, that advice comes with caveats.

Statistics show that 80% of people think that encouraging air from the great outdoors into the building where we now spend up to 90% of our time is a natural and logical thing to do.

Sadly, while indoor air quality has fallen, external air quality is a hazard too; the World Health Organisation (WHO) says 99% of the global population now face air pollution levels that increase health risks which include heart disease, stroke, chronic obstructive pulmonary disease, cancer and pneumonia (

The deterioration is so bad that in 2021 WHO adjusted most of its safe air quality guidelines downwards, in some cases by 75% (

Meanwhile, research in the UK shows that 78% of people believe air near their places of work and homes is “very” or “fairly” clean. This holds for 76% living within a mile of an A-road, 74% near a railway station, 73% close to a motorway, and 70% within the vicinity of an industrial plant.

In fact, the reverse is often true on days of high pollution in surrounding areas to the extent that it is now suggested that looking at local pollution levels must become as automatic as checking the weather forecast.

Locking out pollution

Is living within sealed buildings the answer? Unfortunately not unless many common products, chemicals, and indoor activities that are significant causes of poor air quality are eliminated.

The ultimate goal is for it to be safe to open windows and doors and allow the free passage of fresh clean natural air inwards. However, until this is practically possible there are steps and measures that building owners, occupiers, staff and visitors can take to improve safety.

Post-pandemic changes

The airborne nature of the Covid pandemic spread, particularly in places where pollutants and pathogens could concentrate, linked to a new understanding of what effective ventilation actually means, has altered our relationship with air.

A good starting point is to understand the physical relationship between indoor and outdoor air.

Until now, planning and environmental permitting has assumed that pollution levels measured on the outer facades of buildings — walls and roofs — is an acceptable equivalent measure for airborne risks experienced indoors.

However, the recent sharp focus on the risks that poor ventilation and low air quality can create has increased our recognition that we could well be exposed to far greater risks than previously thought when living and working between four walls for a large part of our lives.

Better ventilation

In practice, outside air can affect inside air via a number of pathways. The first is the passive mechanism by which air can gradually but continuously infiltrate and exit through gaps, cracks, vents, and badly fitting doors and windows in “leaky” buildings.

However, we are also being encouraged to draft-proof properties to cut energy use, and therefore carbon emissions, even though this negates any benefits of natural air flows, such as reducing dampness and mould growth.

One alternative is mechanical ventilation which forces air through buildings artificially. However, this may take us back to square one if unfiltered outdoor air is drawn in. A further option is to add filters, or closed-loop internal cleaning and recycling systems, to eliminate this risk, the caveat being that air flows must be sufficiently large.

With sensors, many modern mechanical ventilation systems are also able to adjust airflows automatically to specific room, corridor and zone specifications. Older systems should be set to prioritise the flow of incoming outdoor air over continuously recirculated air.

Early planning

Designing effective layouts and ventilation systems when new buildings and extensions are still in the planning stage can help to avoid mistakes before they become embedded. Good advice on the use of heating, ventilation and air-conditioning (HVAC) systems is important too.

Further precautions

But that is not necessarily the end of the problem. The use of chemicals, toxic substances and cleaning products listed briefly later in “sealed” buildings can potentially make the indoor environment more perilous than outdoors.

While good mechanical ventilation systems can substantially mitigate their effect, avoiding the use or presence of specific materials, while also adapting maintenance and cleaning schedules, are often crucial to a complete solution.


The Health and Safety Executive (HSE) provides regularly updated best practice advice and information on regulations; it also notes that by law employers must ensure an adequate and unimpeded supply of fresh air in enclosed work spaces.

The government agency offers guidance to help employers and employees:

Pandemic gains

If the pandemic had a qualified silver lining, it could be to precipitate the largest review of urban public health planning in almost 200 years, with recommendations for governments to embed better ventilation in planning codes for public buildings to end the aerosol spread of diseases in crowded indoor spaces.

A historic precedent was London’s 1842 cholera epidemic when Edwin Chadwick managed to secure clean water, improved drainage systems, and a ruling that councils must clear refuse from homes and streets — measures that have led to effective hygiene codes in cities around the world.

It is perhaps easy to regard 19th century Londoners as naïve for thinking that a miasma (smell or vapour) caused cholera, when it took us some time recently to recognise that the indoor risk of spreading modern invisible airborne pathogens can be equally dangerous.

One explanation for this is that it may be relatively ease to identify an individual water pipe or food source as a health problem. Finding a nebulous virus is more difficult. Building designers have also concentrated for decades on keeping people at a comfortable temperature … and saving energy.

New standards

The Building Safety Act 2022 is now enshrined in law as a response to the Grenfell Tower tragedy; in the next 12–18 months it is expected to include ventilation regulations changes designed to both improve indoor air quality and reduce the spread of airborne viruses in new non-residential buildings.

As well as regulating the use of CO2 monitors, it will see additional standards introduced for recirculating ventilation systems in new offices. Existing buildings must also monitor indoor air quality and improve ventilation as needed. Detailed regulations and guidance will be published.

What is clean air?

There has also been a long debate about defining “acceptable” indoor air quality. One approach is how often indoor air is replaced by outside air.

As such, classrooms must have 2.5 changes per hour; carbon dioxide in halls, gym, dining, and exercise spaces should be limited to 1500 ppm. A clean air ventilation rate of 10 litres per person per second is recommended to protect health and wellbeing.

But many buildings fail to achieve this. Poor indoor air quality has also been linked to sick building syndrome, lower productivity, and impaired learning in schools.

Health costs

A specific poor air quality concerns is Particulate Matter (PM1 and PM2.5) that can penetrate lungs, enter the bloodstream, and cause respiratory and cardiovascular morbidity and disease.

Because airborne particles can accumulate rapidly in poorly ventilated spaces, indoor pollution can be up to five times higher than that outside. With circa 25% of hospitals and 33% of GP surgeries in England in areas where safe levels are exceeded regularly, monitoring indoor air quality is important.

The impact of long-term exposure to particulate air pollution is estimated to equate to 29,000 UK deaths annually. A 2017 snapshot of NHS and social care costs resulting from a combination of PM2.5 and NO2 was estimated to be £42.88 million — probably rising even higher to £157 million when less robust evidence is included. Total costs could rise to £8.3 billion by 2035.

The current requirement is for external air to be taken in and circulated to reduce CO2 levels building up during the day. However, incoming air must also be cleaned of nitrogen dioxide (NO2) and PM1 and PM2.5 particles before it is used to dilute, and eventually displace, internal air and its pollutants.

Community research

An Imperial College study is currently exploring how indoor air pollution affects human health indoor air pollution affects human health.

In a series of four-year projects in West London, environmental, social, medical, engineering, economic, scientific, and local health community experts are working together to investigate how the composition, concentration, and exposure to air pollutants affects children with asthma and people in urban homes and workplaces.

Threats lurking in offices and homes

There are simple steps that most people can take to reduce direct health risks in their home or workplace. They include avoiding the use of:

  • Volatile organic compounds (VOCs) — chemicals found in many materials in the home which evaporate into the air at room temperature as vapours we can inhale

  • Particulate matter — minute particles that can enter lungs and create serious health problems

  • Carbon dioxide (CO2) and carbon monoxide (CO) — released from appliances that burn fuel, such as open fires and gas cookers

  • Other pollutants — that can include radon, nitrogen dioxide (NO2), water vapour from condensation, bacteria, mould, fungus spores, dust-mites, and flecks of pet skin or fur (dander).

Building managers are advised to measure and monitor relative humidity (RH), ventilation rates, and the potential build-up of mould, temperature, CO, CO2, VOCs, PM, NO2, NOx and radon.


Most people think opening windows improves indoor air quality. Although that allows bad air out for a while, the advice is to close them again if living or working close to major roads, railways and industrial sites because low outdoor air quality is often also a substantial hazard.

Outdoor air enters buildings through gaps, cracks, vents, and badly fitting doors and windows. However, draft-proofing a building to save energy means this can leave occupants trapped indoors with toxic chemicals, products and cleaning materials.

Mechanical ventilation is an alternative which forces air through buildings artificially. However, if unfiltered outdoor air is drawn in, that still leaves the problem of poor quality external air. Filters, or closed-loop internal cleaning and recycling systems, can eliminate this risk.

Early planning, with effective layouts and ventilation systems when new buildings and extensions are still in the design stage, can help to avoid mistakes before they become embedded.

Precautions to avoid the use of volatile organic compounds (VOCs), particulate matter, carbon dioxide (CO2) and carbon monoxide (CO), and other pollutants must be part of a full solution.

Advice from the Health and Safety Executive (HSE) to help employers and employees maintain a safe and health workplace includes guidance to identify poor ventilation and decide how to improve ventilation.