How pollution affects indoor air quality in buildings

The quality of the air people breathe is vital for their health and wellbeing. CIBSE Journal looks at two reports focusing on how outdoor pollution affects indoor air quality in schools and commercial premises based in and around London

The UCL Institute for Environmental Design and Engineering – in collaboration with the University of Cambridge’s department of chemistry – has published a new report investigating levels of indoor air pollution in London’s schools.

Commissioned by the Mayor of London, the report examined five primary schools and one nursery. It found that children living – or going to school – near busy roads were exposed to higher levels of vehicle pollution, and had a higher prevalence of childhood asthma and wheeze. In addition, research published this month by We Design For has revealed that indoor air quality in buildings tested by the consultancy was worse than outdoor air quality. Premises in central and north London, and in Wycombe were found to be exceeding particulate matter (PM) guidance limits by up to 520%.

Tacking IAQ issues in schools

UCL’s Indoor air quality in London’s schools report evaluated indoor air quality (IAQ) at the primary schools and nursery. It found differences in pollution levels between classrooms depending on a range of factors, including building characteristics, design and maintenance. A significant proportion of indoor air pollution is caused by outdoor pollution.

For nitrogen dioxide (NO2), which is strongly related to the risk of asthma attacks and asthmatic symptoms, outdoor sources accounted for 84% of the variation between classrooms, highlighting the importance of tackling emissions from road traffic and preventing it from entering the building. The findings suggest that the protection offered by the building increases the further away it was from the busiest roads and that airtight buildings may offer greater protection. The report also found that, in most classrooms, annual exposure to small particles was higher than the recommended World Health Organization (WHO) guidelines, although this was caused by a combination of indoor and outdoor sources.

Indoor air quality and temperature in schools – key facts

  1. Evidence indicates that lower temperatures, in the range between 25°C to 20°C, improve student performance by 2-4% for every 1K reduction.
  2. The thermo-physiological (PMV) and the adaptive comfort approach form the basis of the current standards for thermal comfort in schools. Both approaches have been developed from the findings of studies on adult subjects. However, research evidence suggests children in temperate climates prefer lower temperatures than adults. More specifically, comfort temperatures for children were found to be 4K lower than the predicted mean vote (PMV) and 2K lower than the EN 15251 adaptive comfort model predictions.
  3. Higher indoor CO2 levels have been associated with increased probability of communicable infection, asthmatic symptoms, absenteeism, and impaired academic performance of children. For every additional 1L.s-1 per person, illness absenteeism rates in children were estimated to reduce by at least 1%.
  4. Currently, there is no evidence on the effect of VOC levels on cognitive performance of students. Higher levels of total volatile organic compounds (TVOCs) were associated with Sick Building Syndrome (SBS) symptoms and dissatisfaction with IAQ. Control of indoor sources – together with CO2 levels below 1,300ppm (95% confidence interval, CI: 1,200 to 1,400ppm) – may result in TVOCs levels below 200 μg/m3, which is the lowest threshold for discomfort in some countries.
  5. Increased ventilation rates can reduce indoor mould concentrations (an increase of 1 colony-forming unit per m3 of microbial concentrations for every 1ppm increase in CO2); however, thresholds for mould concentrations are not well defined. Sufficient evidence associates high microbial concentrations with general and respiratory symptoms in children.
  6. Traffic-related pollutants, such as particulate matter, ozone and carbon monoxide, were linked to increased illness-related absenteeism and asthma incidence. Exposure to nitrogen dioxide was related to asthma incidence and exacerbations. Indoor levels of traffic-related pollutants should be considered separately, because low CO2 levels do not guarantee a healthy environment, although they offer a first indication of exposure.

Principal investigator Dejan Mumovic, director at UCL Institute for Environmental Design and Engineering, says: ‘Now we have sufficient evidence that children living or attending school near high-traffic-density roads are exposed to higher levels of motor-vehicle pollutants, and this increases the incidence and prevalence of childhood asthma and wheeze. Policy should be directed towards city-wide planning, such as making streets pedestrianised and urban greening around school buildings within 150-250 metres of the major roads.’

A risk assessment of potential outdoor and indoor sources of pollution at design stage can inform the environmental strategy of a building, and lead to implementation of appropriate mitigation measures. Air filtration near busy roads would be necessary to reduce the concentration of particulate matter in indoor air. Activated carbon filters can help reduce concentration of NO2.

Dr Esfand Burman, UCL lecturer in building performance, carried out post-occupancy evaluations in five modern schools constructed under the UK Innovate Building Performance Evaluation programme.

‘It is important to strike the right balance between air quality and energy efficiency in a building,’ he says. ‘While a mechanical solution may help in high-traffic, urban areas, natural ventilation can still bring environmental benefits where outdoor air pollution is not high.’

CIBSE’s best practice guidelines, TM57 Integrated school design (CIBSE, 2015), indicate that a well-implemented natural ventilation strategy could be the default design solution for the ventilation of school teaching spaces when the ingress of external noise levels can be avoided. This is assuming external pollution levels do not exceed established health IAQ guidelines, which is currently not the case across London.

A significantly more stringent regulatory regime for diesel vehicles and measures to reduce traffic around buildings occupied by people who are more vulnerable to air pollution – for example, schools and hospitals – can help cut the outdoor sources of pollution.

The mayor announced actions to clean up London’s bus and taxi fleets, bringing forward the introduction of the world’s first Ultra-Low Emission Zone and introducing the Toxicity Charge (T-Charge) for the oldest polluting vehicles in central London. This might be insufficient, but it is a big step in the right direction.

At building level, a review of existing regulations and guidelines for energy efficiency and air quality could be helpful. An integrated approach to energy and air quality could, for example, offer an allowance to adjust energy efficiency requirements in heavily polluted urban areas in exchange for specific measures to control air pollution. On the other hand, tough energy efficiency requirements where the risk of outdoor air pollution is not high can incentivise natural ventilation and passive measures.

Three steps to Controlling the IAQ in buildings

  1. Manage the sources of pollutants by:
    a. Removing them from the occupied space – for example, regular ductwork cleaning
    b. Isolating them from occupants by means of physical barriers, or by pressure difference. During renovation work that produces dust, for example, seal off inlets and outlets to prevent spread of dust throughout the building
    c. Controlling the timing of their use; during renovation work that produces dust, do not operate the heating and cooling system until after cleaning up the dust.
  2. Use purge ventilation to dilute pollutants and remove them from the building. For example, always ventilate when using products that can release pollutants into the air, such as paints and lacquers, paint strippers or varnishes.
  3. Use filtration, if necessary, to clean the incoming air. In high traffic areas, for example, installation of filters with low resistance to airflow filtering up to 95% of PM10 and PM2.5, and removal of nitrogen dioxide.

Dr Lia Chatzidiakou, research associate at the University of Cambridge, and Dr Joe Williams, researcher at Feilden Clegg Bradley Studios, who carried out research on indoor air quality in schools while at UCL, say the mayor’s office has offered funding to 50 schools and nurseries in high-pollution areas to help improve indoor air quality. ‘It is essential to set up a scientifically robust study to evaluate the effectiveness of measures that are implemented.’

‘The majority of studies on indoor air quality so far have focused on CO2 measurements as an indicator of IAQ, reflecting the difficulty and expense of obtaining measurements of specific pollutants,’ adds Mumovic. His top tips are:

  • If your school is on a busy main road, ask your local council to evaluate its IAQ. Installation of a mechanical ventilation system with particulate and gaseous pollutants filtering might be required. Also, check if it is possible to introduce no-vehicle zones around your school.
  • Consider replacing carpets with low-emission hard flooring, which is easier to clean and not prone to becoming a source of microbes and allergens. As children move around the classrooms, unsuitable finishing elevates indoor PM concentrations through resuspension of previously deposited particles. Reconsider fleecy materials in school classrooms: no upholstered chairs, teachers’ armchairs or fleecy notice boards – use cork pinboards instead. Avoid storing books on open shelves in classrooms: use closed cabinets in corridors.
  • Store cleaning and artwork materials outside the classroom to eliminate indoor sources of air pollution. Switch to low-emitting cleaning products and paints.

IAQ in commercial buildings

The research by We Design For – which was launched by three former Battle McCarthy colleagues – has revealed London’s indoor air quality may be worse than its outdoor air quality. Data from the study shows indoor air standards in some buildings is failing to comply with WHO guidance and Building Regulations.

Policy should be directed towards city-wide planning, such as making streets pedestrianised and urban greening around school buildings within 150-200m of major roads

We Design For has recorded air quality in a number of locations around London since September 2017, and its data shows indoor air quality in public and private spaces – such as commercial offices, retail spaces, residential houses and apartments – is consistently poor. In many cases, it is worse indoors than outside – meaning fresh air supplies are far from fresh – and could pose a long-term health risk.

The research found that the studied properties – in central and north London, and Wycombe – are exceeding PM guidance limits by up to 520%, and it is unlikely that many comply with the Building Regulations’ annual mean limit for NO2. More than 60% of the buildings tested in the past year had higher levels of harmful NO2 and PM2.5 inside than out.

Building Regulations state that exposure to nitrogen dioxide should not exceed 288 μg/m over a one-hour average, and 40 μg/m over a long-term average. Many buildings are failing to satisfy these targets, yet there is rarely monitoring in place to measure, record or tackle this issue.

All buildings are required, by law, to offer a level of ventilation to help reduce the risk of damp and condensation, prevent the build-up of bacteria and remove allergens and pollutants. If the air being brought into homes and offices is not fresh or clean, however, the process can become counterproductive.

Dr Dominic Clyde-Smith, from UCL – and head of research at We Design For – says: ‘The test results we have been gathering over the past year show indoor air pollution is a serious problem within our homes, offices, shops, schools and other premises.

See results on the We Design For website

‘Most interestingly, our test results have highlighted that ventilation alone is not a viable solution – recognised industry guidance regarding the control of ventilation systems and location of fresh-air intakes needs updating.

‘What we are seeing in London is conclusive evidence that the ventilation and filtration systems simply aren’t up to the task, leaving Londoners to breathe in pollutants and harmful particulates at levels that are far from safe.

‘As an industry, we need to design for appropriate levels of filtration and look to reduce sources of air pollution. This is a problem that we are very capable of solving.’

Pete Carvell, director at We Design For, adds: ‘People tend to believe that the widely publicised effects of air pollution do not affect them when they are indoors – either at home or at work; they assume the air indoors is cleaner than on the street. Our results prove this is not the case and that conditions indoors are often worse.

‘Urban dwellers need to be asking more questions about their indoor air quality. We need to be looking at what we can do to make indoor air quality better, just as we work to reduce outdoor air pollution.’

For details, see a selection of the data at

A detailed summary and report can be read at