Pathology: indoor air quality

The enemy within

11 August 2011

Highly engineered buildings might meet tough environmental standards, says Trevor Rushton, but they can also increase the health risks to the people in them – including those from house dust mites, mould and spores

To achieve the UK government’s targets for zero carbon in residential construction strenuous efforts are needed to limit heat loss and uncontrolled air leakage from buildings. Indeed, homes built to Level 4 and above of the Code for Sustainable Homes require mechanical ventilation and heat recovery system (MVHR) to provide a satisfactory indoor environment. Over the coming years, we will see more and more housing with this type of equipment. But the provision of MVHR has a number of issues – some cultural, some practical and some medical.

First, as a nation we are used to fresh air – throwing the windows open to give the house a good blow through creates a sense of wellbeing. Yet, it is this uncontrolled air leakage that new standards are designed to reduce. Joe Public may struggle to accept the change, but to meet targets it is no use simply creating airtight buildings; we need to shift the culture of the people using them. This is possibly a greater challenge – we can develop methods and machines to deal with a problem, but a cultural shift is harder to encourage.

Keep it simple


It has been estimated that air pollution reduces the life expectancy of every person in the UK by an average of 7-8 months, with estimated equivalent health costs of up to £20bn each year

Second, MVHR systems need to be simple to understand and maintain, with parts readily available and with clear instructions for their proper function. According to research by the NHBC Foundation (Ref. 1), many systems are selected based on achieving the background and purge rates set out in the Approved Documents rather than their long-term use.

Complicated control systems are unlikely to be satisfactory – the first user may have a maintenance pack, but documents become lost and users may not possess the cognitive skills to understand the need or the proper function of the system.

In addition, mechanical equipment needs servicing and parts eventually wear out. More often than not, the gradual deterioration in performance goes unnoticed until the equipment breaks down altogether or is simply turned off; back to square one again on the air leakage front.

A further consequence will be deterioration of indoor air quality and the attendant health risks. It has been estimated that air pollution reduces the life expectancy of every person in the UK by an average of 7-8 months, with estimated equivalent health costs of up to £20bn each year (Ref. 2). While we have known for years that air quality is important, surprisingly little research has been undertaken in the UK to investigate the effects of airtightness on highly insulated homes and their occupants.

Breathe deeply

Sceptics have long argued that “buildings need to breathe” and that making them airtight can only have bad consequences – as we continually raise the green bar, the truth of this statement will become apparent. It isn’t that the problem is not recognised, but that as the building becomes more highly engineered so the ‘physics of building’become more critical. The margin between success and failure is much narrower and if care is not exercised in the design and construction stages as well as the occupation, unintended consequences will occur, e.g. condensation, mould and rot.

Current building practice usually incorporates an impervious vapour barrier on the warm side of a room; this will help prevent the entry of contaminants from the outside, but also serves to trap contaminants from inside and, if ventilation is inadequate, permits increases in their levels.

According to The European Commission Scientific Committee on Health and Environmental Risks (Ref. 3), studies have shown there are around 900 harmful or potentially harmful chemical substances or biological contaminants in the air we breathe. The table below summarises some of the more important ones.

The Approved Documents to the Building Regulations (Part L dealing with airtightness and Part F dealing with ventilation) specify air quality standards for a limited number of substances only and place greater emphasis on airtightness than ventilation. They assume that outdoor air is ‘fresh’ where the reality is that in some areas (e.g. Greater London) it is likely to be contaminated by industrial and transport emissions. There is therefore a case for better guidance and regulation of indoor air quality, including more efficient ventilation standards and those for indoor pollution levels (Ref. 4).


The house dust mite is just one of the biological problems caused by poor indoor air quality

Missing labels

Indoor emissions can be limited by thorough and regular maintenance of heat-producing appliances and by ventilation systems. Flueless gas appliances are a major source of particulates, carbon and nitrogen oxides and are best avoided. Similarly, carefully selecting furniture, soft furnishings and decorative products can limit VOC emissions. All building materials should be labelled to demonstrate their VOC emissions (or the content of other potentially harmful chemicals) but in the UK this is not the case.

Clearly, managing these risks requires a heightened awareness of the issues – while many people will know of the risks of outdoor pollution, awareness of the perils lurking within the home is far lower. In new housing, the supply chain needs to be geared towards limiting the effects of pollution and ensuring that the properties of the materials used and the systems installed can deal with the effects of emissions.

This will not simply be achieved by the provision of MVHR, but by the careful design and selection of systems that can be properly maintained and which are able to perform better than the minimum standards currently set under the Building Regulations. Importantly, end users need to be better educated about the potential effects of poor indoor air quality and the need for regular maintenance of equipment and regular indoor cleaning.

Some potentially harmful chemical substances or biological contaminants

Pollutant Sources and health impacts
Volatile organic compounds (VOCs) Arising naturally and from furniture, building materials, paints, floor coverings, etc. Not all VOCs are harmful but some exposures can lead to sensory effects, and high levels can cause general discomfort due to strong smells, irritancy or allergic reactions, headaches, nausea, dizziness and drowsiness. Some VOCs are known carcinogens. Microbial VOCs are released from the metabolism of fungi: they include ethanol and a range of alcohols and ketones.
Ozone
 A strong oxidising agent that can damage human health at high concentrations; it is not emitted as a pollutant but its atmospheric concentration is raised by photochemical reaction involving VOCs and nitrogen oxides.
Bio effluents
 Emitted from the human body by respiration, metabolic processes and bacterial decomposition of the skin and comprising 100s of VOCs in low concentrations along with carbon dioxide. Together, they contribute to human body odour that is not toxic but has a number of sensory effects.
Environmental tobacco smoke (ETS)
 While a UK ban on smoking in public places was introduced in 2007, smoking in the home is a major source of ETS components, of which some are known carcinogens. Thought to account for around 11,000 UK deaths every year from lung cancer, stroke or heart disease (Ref. 5).
Water vapour
 This is water in a gaseous form produced by activities such as washing, breathing, cooking, etc. The quantity has a direct effect upon indoor air conditions and can facilitate a variety of biological problems, such as house dust mites, mould spores, etc.
Products of combustion (cooking, heating, etc)
 These are generally carbon monoxide and nitrogen dioxide and, in the case of coal burning, sulphur dioxide. Carbon monoxide is lethal at high levels and has potential chronic effects at low levels. Nitrogen dioxide from heating/cooking appliances is associated with respiratory symptoms.
Airborne particles (particulate matter)
 For example, asbestos fibres from disturbed insulation or building products, lead from paint, other particles released during cooking (superheated oil particles) or from aerosols. Associated with reduced lung function and increased risk of heart and respiratory disease.
Particles of biological origin
 There are four main categories: pollen, fungal spores, faecal pellets of the house dust mite and bacteria. All can potentially cause serious health effects or allergic reactions.

  • house dust mites tend to reproduce in conditions of high ambient temperature and humidity – typically in late summer
  • airborne particles originating from pets can cause allergic reactions
  • microscopic spores from fungi are released into the air to spread the fungus. Microscopic hyphal fragments, or cells of surface-growing fungi such as moulds or yeasts, may become detached and thus airborne
  • pathenogenic particles from occupants or water sprays can lead to a worsening of symptoms of asthma and cause wheezing.
Penetrating ground gas
 For example, radon, which is radioactive and associated with lung cancer, or methane from landfill.

Trevor Rushton is a Director of Watts Group, a regular contributor to isurv and a member of the Building surveying editorial board.

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