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How can designers tackle indoor air pollutants?

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Alastair Lewis

The University of York’s Alastair Lewis explains everything architects need to know about indoor air quality – the risks and solutions – and Enhabit’s hypoallergenic house example in Richmond shows how

Barely a day goes by without one of the major news outlets reporting on the dangers of air pollution. Increasing numbers of parents are campaigning to end car-idling outside schools, which is being linked to premature deaths, and cities all over the UK are introducing low emission zones. Exposure to air pollution has rightly become one of the most high-profile issues affecting public health.

However, while most media reporting focuses on the quality of air outdoors, little attention is given to the effects of exposure to pollution indoors despite the mounting scientific research in this area. Yet people spend on average up to 90% of their time inside, so indoor air quality is a critical part of managing health, as the Royal College of Physicians and the chief medical officer recently emphasised.

Historically, indoor air pollution often mirrored what was going on outdoors. When homes and offices were poorly insulated, pollution blew in through windows and doors and gaps in between. But over the past three decades there has been a profound change. UK outdoor pollution has improved considerably while air exchange in buildings has been reduced, often with energy efficiency in mind. Links between interior and exterior air quality are broken, so responsibility for indoor air quality now lies largely in the hands of building designers and, critically, the occupants themselves.

Most of the important outdoor air pollutants, including particulate matter, can also be found indoors, with the same underlying biological mechanisms for affecting human health. Particulate matter is most commonly measured as particles smaller than 2.5 microns in diameter or PM2.5 and includes nitrogen dioxide (NO2), carbon monoxide (CO) and a range of volatile organic compounds (VOCs). Short term, such pollution can irritate the lung lining triggering respiratory symptoms and placing acute stress on the cardiovascular system. Long-term exposure is being increasingly implicated in greater risk of cancer and neurological diseases such as dementia and Parkinson’s. While ozone is an important pollutant outdoors, it is not found in substantial concentrations indoors because, as a reactive species, it is readily destroyed on indoor surfaces. Few UK homes now burn high sulfur-content coal, and as a consequence SO2 has largely disappeared indoors too.

So how is air quality inside caused and controlled? At the Healthy House in the case study shown in the images and outlined below, we helped Enhabit carry out the monitoring of indoor pollutants once complete, but before that in the design process, the answer lay and lies in three major processes: ingress, embedded emissions and occupant activity. The first of these, the ingress of pollution from outside, is largely a function of the air exchange rate, and where that fresh air is drawn from. In most domestic settings the air exchange process is passive, effectively air leaks, while in commercial buildings it is most likely engineered in. Avoiding urban air intakes within street canyons is one simple positive move, since the highest concentrations outdoors are typically found roadside and decline rapidly with building height.

The second major factor controlling indoor air quality is the fabric of the building itself. The most widely reported embedded pollutants are VOCs, chemicals that are a component of a wide range of construction products (wood composites for example), are in furniture via glues, in floorings and carpets, and in decorative materials like sealants, paints and varnishes. The embedded VOCs within a building slowly decay over time as the chemicals evaporate and react, and that’s why in the Richmond house its owners decided to buy a second-hand sofa rather than a new one, but this process can take many years and over the intervening period individuals with high sensitivities can experience acute symptoms. Use of low VOC content materials – such as water-based paints – helps reduce the scale of impact, and so may temporary increases to ventilation rates early in a building’s life.

Other embedded sources include primary fossil fuel heating systems that, while exhausted on the outside, lead to residual internal pollution. Internal connections to attached garages, for example, cause pollution from leaking fuel tanks to seep indoors. Not all embedded processes are negative of course. Ventilation over cookers can help reduce particulate pollution and there is growth in the adoption of active air filtration. The latter is, however, not without its own effects, creating new energy demand and potential toxic waste products.

The final and highly variable source of indoor pollution, particularly in homes, arises from occupant lifestyle and activities. Air quality in the home is largely a consequence of many small individual choices coupled to the building air exchange rate, and individuals have a substantial degree of autonomy and control over both factors.

In most UK locations away from busy roads, outdoor air is likely to be cleaner than indoor air for some classes of pollution. In these circumstances reducing the indoor emissions of pollutants, for example by changing patterns of combustion and chemical consumption, can lead to direct reductions in exposure, as can actions that increase ventilation through simple actions such as opening windows more frequently.

  • The front door of Enhabit's Healthy House in Richmond. The house is designed to be sealed from external pollutants.
    The front door of Enhabit's Healthy House in Richmond. The house is designed to be sealed from external pollutants. Credit: Rooz Photo
  • The house is half sunk below ground level, with bedrooms at basement level and a planted staggered garden around it.
    The house is half sunk below ground level, with bedrooms at basement level and a planted staggered garden around it. Credit: Rooz Photo
  • The Healthy House exterior is rendered with timber rainscreen cladding.
    The Healthy House exterior is rendered with timber rainscreen cladding. Credit: Rooz Photo

Perhaps the most significant impact on domestic indoor air quality and exposure comes from the discretionary use of solid fuel burning in decorative stoves. This has grown to become a major outdoor pollution problem in some UK cities and the Government's 2019 Clean Air Strategy places domestic wood-burning in the policy cross-hairs. There are many other discretionary activities that lead to indoor emissions – frequency of use of cleaning and personal care products, hobby solvents, fragranced materials and even printer cartridge ink and burning of candles. While UK emissions of virtually all other classes of pollutants have declined over the last 10 years, the exception is the domestic consumption of VOCs, which alone shows an upwards trend.

Most VOCs released indoors are of low direct toxicity, but some individuals do exhibit a greater sensitivity than others. In a small number of cases these lead to genuinely debilitating symptoms. Since indoor air is increasingly ‘locked in’, particularly in smaller energy efficient homes, it also becomes increasingly important to consider the whole life-cycle of the chemical, not just the initial toxicity.

VOCs are chemically reactive and over a period of a few hours to a few days are oxidised in a process that is analogous to a low-temperature flame, ultimately producing CO2 and water. This is a complex multi-step process however and some of the by-products created can condense and form particle pollution, while others may stick to internal surfaces. This oxidation process breaks VOCs apart producing in some cases secondary chemicals with known human toxicity such as formaldehyde and acetaldehyde.

The construction industry have been one of the most forward thinking in terms of awareness of VOCs as pollutants, and this is the only sector that voluntarily labels its consumer products. Paints and varnishes have for many years marked up VOC content – the globe symbol, pioneered by B&Q, is now used almost universally. There are also a range of formal regulatory requirements on VOC emissions set down in EC Building Products and Paints Directives. But professional management of VOC indoor emissions through selection of building materials can only go so far and must be set against this rising tide of domestic discretionary consumption.

There is ultimately a shared responsibility for maintaining good indoor air quality. Building design needs to ensure that energy efficiency goals do not conflict with or override the need to ventilate with fresh air. While VOCs are impossible to completely avoid during the construction and furnishing of a building, individual choices made by the developer have measurable impacts on the health of occupants, both those with direct sensitivities and longer term to everyone. Designing in known sources of indoor pollution such as gas heating or solid fuel stoves creates the potential for long-term harm to occupants that can be avoided.

Owners and occupants themselves play a critical role and ultimately have most of the controls needed within their hands. Perception and knowledge of indoor air quality remains low, with a prevailing view that pollution is overwhelmingly from traffic at the roadside. Clearer guidance on how to live healthily indoors is needed, defining better what creates pollution, and how it can be straightforwardly managed. Substantial gains can be achieved with simple actions like reducing or eliminating unnecessary combustion and moderating solvent consumption, using built-in mitigation such as extractors, and treating some active ventilation as a positive and necessary requirement.

Alastair Lewis is professor of the National Centre for Atmospheric Science at the University of York

  • The Healthy House uses natural materials and second hand furniture, including the sofa, to minimise the VOCs emitted.
    The Healthy House uses natural materials and second hand furniture, including the sofa, to minimise the VOCs emitted. Credit: Rooz Photo
  • The hallway of the Healthy House.
    The hallway of the Healthy House. Credit: Rooz Photo
  • Fittings, including the kitchen units, were selected for low or no VOC content.
    Fittings, including the kitchen units, were selected for low or no VOC content. Credit: Rooz Photo
  • View looking back from the living room through to the hallway and kitchen, designed by Enhabit.
    View looking back from the living room through to the hallway and kitchen, designed by Enhabit. Credit: Rooz Photo

Case study: Richmond Healthy House, London

Designer: Enhabit

Elinor and Born came to see us even before they had purchased their plot of land in Richmond, west London. Their ambition was clear: they wanted to build a healthy home for their family of five, given that two of their children were severe asthma sufferers. They also wanted to ensure that the project had a limited impact on the environment and was cheaper to run. They had a very fixed budget and a tight timetable. The planning approval was for a basement and single storey, so we had to work within these constraints.

The first priority was the practicality of the build. Access to the plot for construction work was due to last only four months, so we needed to get the building up as quickly as possible. We therefore recommended using a SIPs system to minimise construction time.

Next, the detailed design. The brief was a healthy home – but what did that mean? Air quality was clearly important, so the priority was to minimise indoor pollutants. The first step was to see if we could come up with materials which contained no or very low levels of VOCs. This isn’t as easy as it sounds – anything that includes glue will emit VOCs, so we had to look carefully at all the racking boards, structural timber and finishing materials.

We did pretty well on this, but we knew we would never get everything to be VOC free, so the complementary strategy was to design a good ventilation system. A mechanical ventilation and heat recovery (MVHR) system was the obvious choice.

All air entering the building is filtered and the air in the building is fully exchanged every three hours. The heat recovery element also means that it is very energy efficient. We capped all this off with low u-value walls/floor/roof and triple glazing to ensure good levels of thermal comfort. We also went for an airtightness target of around 1 air change per hour (ACH) @50Pa as we wanted to minimise external pollution entering the building. Achieving this level of airtightness was made easier because of the SIPs construction.

This resulted in a very low heating demand so we were able to propose an air source heat pump which met the client’s brief for a low-carbon build.

Once we started on site, the basement build went smoothly and was completed just in time for the SIPs panels to arrive. The frame was completed in six weeks and the air test achieved 0.5 ACH @50Pa on its first attempt.

The final result is a beautiful home that undoubtedly works for its occupants. Elinor and Born tell us the kids rarely have any asthma attacks at home and that the house is always fresh and bright, but warm and cosy. It’s nice to have had the opportunity to design something holistically which really works on all levels.

Akta Raja, director/co-founder, Enhabit. The Healthy House has its own website where you can find out more including lists of suppliers.


  • One of the bedrooms at basement level.
    One of the bedrooms at basement level. Credit: Rooz Photo
  • One of the children's bedrooms. The house is designed around their allergy needs to improve asthma symptoms.
    One of the children's bedrooms. The house is designed around their allergy needs to improve asthma symptoms. Credit: Rooz Photo
  • Master bedroom.
    Master bedroom. Credit: Rooz Photo


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