British cities – and London in particular – break the law on levels of air pollution on a near-constant basis. And people are increasingly aware of what a serious risk this poses to health. Respiratory illness, heart disease, dementia – and even reduced thinking ability and increased crime rates – have all been linked to air pollution.
The issue shot to prominence in the public mind after the death of a nine-year-old South London schoolgirl, Ella Kissi-Debrah, was linked to air pollution levels near her home. Last year one of the UK’s most senior doctors, Southampton University’s Professor Stephen Holgate, reviewed Ella’s medical records and air pollution records, and found a “striking association” between her emergency hospital admissions and recorded spikes in pollution. His report said it was very possible that had there not been unlawful levels of air pollution near to her home, Ella might not have died.
Just this October, Imperial College London published new research which showed measurable increases in heart attacks, strokes, and hospitalisation of children with asthma during the worst pollution days in eight British cities.
Political action such as London’s Ultra-Low Emission Zone (Ulez) – which makes the most polluting vehicles pay an off-putting charge if they enter the centre of the city) are gradually bringing down outdoor pollution levels in some of the worst-affected areas. But progress is painfully slow, with more than 1800 locations in the UK exceeding EU legal limits in 2018.
Are we safer inside our buildings?
During the worst pollution episodes, sufferers from lung and heart conditions are urged to stay indoors. But given that the air in buildings originates outside, how much safer are they – are we – indoors?
Does the way a building is constructed make a difference? And could airtightness help?
The outdoor air pollutants of concern are gases and microscopic airborne particles, so it is not surprising to discover that outdoor air pollution does find its way into buildings. Research has confirmed that indoor air quality is “undoubtedly affected by outdoor air” and “in urban areas, a significant proportion of indoor air pollution is due to penetration through the building facade of pollutants generated outdoors”.
This being the case, is there anything we can do to protect building occupants?
Airtightness keeps pollution out
In 2016 the Mayor of London commissioned research into air quality in London’s schools, and the impact of outdoor air pollution on the indoor school environment – schools being particularly important as children are especially vulnerable to harm from polluted air.
The team, led by Professor Deyan Mumovic from UCL London, measured indoor and outdoor NO2 pollution levels in six schools and nurseries. They found that indoor NO2 pollution levels tended to be lower than outside – but the level of protection offered by the building depended on its airtightness.
In the three relatively airtight contemporary schools, indoor NO2 levels were just a half or even one third of levels outdoors. However in naturally ventilated in Victorian schools with old wooden windows, indoor NO2 was only 10-30% lower than outdoors.
This finding was echoed in research from China by Zhen Peng and Wu Deng from the University of Nottingham in Ningbo, again in schools. This team looked at another outdoor pollutant, particulate matter, which in China originates from burning coal as well as vehicle engines. Looking specifically at the larger particles, PM2.5 and PM10, the researchers found that buildings with better air tightness showed relatively greater reductions in particulate matter indoors. Reductions of around 30-50% compared to outdoors were seen in the more airtight buildings, compared with only a 10-15% reduction in the leakiest.
The authors point out that outdoor air pollution penetrated even school buildings which meet the air tightness level required by the latest energy saving regulations in China. “The current building air tightness requirement should be improved [and] retrofits are needed to improve the indoor air quality,” they say.
The role of mechanical ventilation
While improved building airtightness can protect occupants from outdoor pollution, ventilation is still required to flush indoor pollutants away. School managers may feel they are in the invidious position of having to “juggle opening their windows for fresh air with letting the pollutants in”. However, a closer look at the research suggests that it may be possible to get the best of both worlds. By taking control of the air that comes in, by using well-designed mechanical ventilation, designers can add further levels of protection from outdoor pollution.
The London researchers found that levels of pollution can vary significantly around a building. Traffic-related pollution tends to concentrate low down on the street-facing side of a building, and on the sides of the building facing away from the wind.
By selecting less polluted locations for ventilation air intakes, a ventilation system can supply less polluted air than that that would leak in though a facade facing the street, as Professor Mumovic and colleagues point out.
In one school studied by the team, there was a mechanical ventilation system with intakes on the roof. (The mechanical ventilation had been specified to enable the school to keep windows closed, as the location was noisy.) The ventilation system had previously been performing poorly, so had recently been cleaned and re-commissioned. The researchers found levels of NO2 and particulate pollution (and also ozone, O3) were significantly lower inside classrooms than they were outside.
By contrast, poorly sited ventilation intakes can make things worse. There are for example a number of reports of diesel exhaust being extracted into a mechanically ventilated building from unloading trucks when fresh air intakes are sited at low level.
Can filters add extra protection?
A further level of protection can be added by filtering the intake air, as was proposed by the authors of the Chinese study. Most mechanical ventilation systems already include a coarse filter, in order to keep dust and insects out of the ventilation mechanism. However, much finer grade filters are also available, designed to cut out a high proportion of those polluting particles that are too small even to see – such as pollen, and PMs 1-10.
A study in Amsterdam investigated the effect of fine-grade filters in ventilation systems in a high school near the Amsterdam ring road. Their findings confirmed that a fine F8 filter could improve indoor air quality. PM10, PM2.5, and black carbon (the sooty element of particulate matter, derived from combustion) were measured, and all were reduced by around one third.
The research involved replacing an inadequate mechanical ventilation system with a new one. As well as cutting particulate levels, the new system increased air exchange rates overall, leading to a lowering of CO2 levels during lessons. It was also a much quieter system than the less effective one it replaced.
The authors stressed the importance of regular maintenance:
“Our results indicate that the application of a mechanical ventilation system with a fine filter can reduce classroom exposure to traffic exhaust at hot spot locations. However, filtration is only effective if the filters are frequently replaced and the ventilation system is properly maintained.”
Build tight and ventilate right to keep pollution out
In order to achieve the maximum effectiveness of filtered ventilation system, the building envelope needs to be airtight enough to prevent the filter being bypassed by polluted air leaking in though the building fabric. As Tom Heywood, MVHR Design Manager at the Green Building Store, explains: “The level of external pollutants in a building with natural ventilation will completely depend on how leaky it is, and where the air is infiltrating from.”
“The more airtight the building is, the easier it is to control the indoor air quality, providing that there is a ventilation system and it does have filters. If a building is more airtight then less pollution will be coming in through infiltration.”
Julie Godefroy, Technical Manager at CIBSE (the Chartered Institute of Building Services Engineers) agrees that filtering will be more effective in airtight buildings, adding that the use of filtered ventilation will still bring some benefit, even if you are unable to achieve Passivhaus levels of airtightness.
Air pollution harms and kills people, especially the young and those already infirm. The first priority must of course be to reduce and remove the problem at source by social and political change.
Small changes in the immediate environment may help a little. Schools can lobby local authorities to re-route busy traffic, and exhort parents not to drive up close or wait with idling engines. Planting trees and shrubs around homes, schools and other buildings can also filter out some pollution.
But as building users generally have little or no immediate control over outdoor air quality, it is important to offer as many layers of protection as possible, for the many hours people spend inside.
What can construction teams do to protect building occupants?
- Designers: design for high airtightness and provide for good, ducted mechanical ventilation, to control how much outdoor air gets in. Talk to the M&E team about optimising duct layouts for IAQ.
- M&E design and installation team: site air intakes carefully (eg high up), avoiding the most polluted locations. Ensure system capacity is ample for operation with windows closed (except for occasional purges). Consider upgrading from the basic filters to fine filters – and ensure the system is adequate to work well and quietly with filters in place.
- On site: The airtightness champion is looking after health as well as comfort and energy! Make sure the programme is thought through, to maintain the integrity of the protective airtightness layer.
- Building managers: Understand and maintain your ventilation system, and ensure regular filter changes are carried out.
Sadly air pollution will continue harming and even killing people in the UK’s cities for some years to come. The construction industry is responsible for the health and wellbeing of the people using the buildings it builds. Where occupants are threatened by air pollution, the industry has a moral responsibility to build homes and workplaces that do as much as possible to keep that pollution out – by building tight and ventilating right.
Main outdoor pollutants of concern – and their sources
Particulate matter (PM)
Particulate matter is a complex mixture of minute particles and droplets suspended in air. Particulate pollution can harm our heart and lungs – it is linked to asthma, Alzheimer’s disease, cancer and a number of other health problems.
Research shows that particles with a diameter of ten microns (PM10) and smaller can be inhaled deep into the lungs, and smaller particles can penetrate into the bloodstream and reach every organ in the body. The smaller PM2.5 particles can have a particularly bad impact on health.
Sources of particulate pollution include combustion, for example in diesel and petrol engines, woodburning stoves, oil boilers, aviation, and non-road mobile machinery such as cranes and generators. Particulate pollution also includes dust from vehicle brakes and tyres, from construction sites, and from agricultural and industrial operations, even from far away – particulate pollution may be carried on the wind for hundreds of miles.
Nitrogen dioxide (NO2) and Sulphur Dioxide (SO2)
NO2 is a reactive gas formed when fossil fuels or organic matter are burned. At high concentrations, NO2 inflames the airways and can trigger asthma attacks. Long-term exposure may even increase the risk of premature death.
Within London, the largest source of NO2 (along with the other oxides of nitrogen, collectively known as NOx) is road transport with diesel vehicles the greatest contributors to road transport air pollution. Gas burning, including CHP plant, is also a significant source. Many other cities are also badly affected.
Sulphur dioxide (the cause of ‘acid rain’) has similar effects on the body to NO2. SO2 levels have reduced in the UK as less coal is burned, but port cities such as Southampton, Dover, Liverpool and Aberdeen are still affected by this pollutant. Maritime fuel is dirtier than road transport fuel, and ships’ engines are subject to less regulation. Maritime fuel is often burned continuously by ships in port to power their generators, leading to significant particulate, NO2 and SO2 pollution.
Information from the Greater London Authority, Greenpeace, the University of Southampton and Cancer Research UK, and the International Council on Clean Transportation