Outdoor air quality has been receiving rather a lot of attention recently.  The new UK air quality strategy was published in January 2019 and the comparable document in Scotland, published in 2015, is currently under review.  

The main focus is certainly and unsurprisingly on outdoor air pollution since the UK is currently awaiting the outcome of a European Court of Justice referral for failing to meet the terms of the European Directive on ambient air quality.

But we all spend around 90 percent of our time indoors – at home, at work, or in school.  So surely our greatest exposure to possibly harmful air pollution may well be in the home? What might be the risks, and is there much we can do about it?

Assessing risk 

Worldwide, air pollution is estimated to be the fourth most serious risk of attributable health impact and this risk is about equally divided between indoor and outdoor air pollution. 

In fact, indoor air pollution in isolation presents as the 7thmost important risk, above both unsafe water and unsafe sex.

The World Health Organisation (WHO) attributes 4.3 million deaths annually to household air pollution.  In western societies, the risk factor is relatively much less although WHO still reckons that 120,000 deaths are caused each year in Europe by indoor air pollution – stacking up as 3 percent of all heart disease, 3 percent of chronic pulmonary disease, 3 percent of stroke and 2 percent of lung cancer fatalities.  

The WHO has estimated that the economic costs of deaths from air pollution, outdoor and indoor, amounts to 3.7 percent of UK national GDP, or over £70 Bn each year – so this is a very costly problem. 

Is anything being done?  Well, the European Commission established an indoor air quality working group in 2006, which was disbanded in 2012.  The European  seventh Environmental Action Programme in 2013 calls on the EU to develop an indoor air quality strategy – but, as yet, nothing further has happened.  

Determined action 

Independently, however, France has adopted an action plan; Italy has established its own indoor air quality standards for schools; and Finland has adopted its own general indoor air quality standards.  

The EU Directive on energy performance of buildings could eventually require new EU building regulations to adopt mandatory indoor air quality targets but, as yet, there are no measures in place beyond a requirement “to address the issues of healthy indoor climate conditions”.

This is despite very determined action on passive smoking. Early action was taken with smoking bans in aircraft from the 1980s, and now all EEA countries have some bans in place on smoking, but only in public places.

Indoor air quality can’t be considered in isolation, and is clearly related to the ambient outdoor air quality.  There are many challenges with harmful outdoor air quality but indoor air quality can, rather often, be even worse.  Equally, for some pollutants, it can sometimes be better. The relationship is complex.

The common pollutants, found outdoors and indoors at impactful concentrations, are nitrogen oxides, sulphur dioxide, ozone and particulate matter (PM).  The pathways between outdoor and indoor environments are classified as through mechanical ventilation (air conditioning), natural ventilation (opening windows and doors) and infiltration (leaks around window and door frames, and through roof space).  

Anthropogenic particles

Typically, houses in the UK seem to show that about a third to a half of indoor air is exchanged between inside and outside each hour, but a huge range is found of about 10x between the most leaky and the most air-tight houses.

The permutations found across different buildings and with different outdoor conditions, of say wind strength and direction, can begin to produce hugely variable observable results.  

However, studies have shown that around 50 percent to 75 percent of the variability of indoor air quality of these common pollutants, is explained by outdoor variation.  If pollutants enter a building they can subsequently, to some degree, be diluted or concentrated by specific indoor conditions and activities.

Let’s look, first, at airborne particulates.  These come in various forms – some of which may well be natural, such as fine sand picked up from the Sahara Desert and transported on southerly winds to the UK, or volcanic ash blown here from Iceland. 

But, more generally, particulate matter is created by burning fossil fuels and, especially in urban areas, derived from vehicle exhaust fumes and dust created from vehicle brake-pads and tyres. 

These anthropogenic particles are oily-based and contaminated with numerous exotic chemicals.  Physically, a range of sizes of particles is created. From fairly large, known as PM10, indicating they have diameters of 10 microns or less (for comparison, a human hair has a diameter of about 50 microns) to very small, known as PM2.5 with diameters less than 2.5 microns.

Physiological effects

The physiological effects of these different particles are important:  larger particles (>10 microns) are filtered out in the nose and upper pharynx, while smaller particles (around 1 to 2 microns) reach deep into the alveoli, the air sacs, of the lungs and can cause much more severe damage.

The physical behaviour of these particles is also important. The fall velocity of particles of 1 micron diameter is as little as 0.1 mm per second.  Natural air movement in any room is likely to well above this value, so the particles never settle out.  In fact, it’s generally thought that particles below 10 microns will usually stay suspended indefinitely.

The ratio of indoor to outdoor air quality is found not to be lower than 0.7 for PM10 when there is no additional harmful indoor activity.  This suggests a small benefit of being indoors when high levels of PM10 are encountered outdoors.  

However, it is the finer particles, PM2.5, that are more dangerous to human health and these are not much reduced by being indoors, unless there is specific mechanical filtering by air conditioning units.  

It’s worth remembering that, for example, around 95 percent of the population of London are routinely exposed to harmful outdoor concentrations of fine particulates. Sources of additional particles, produced indoors, can include cooking and malfunctioning gas heaters or wood/coal burners, thereby adding to indoor concentrations.

Internal pollutants 

In contrast, at times of high outdoor pollen levels then these much larger particles (pollen is generally between 10 micron and 100 micron) are greatly reduced indoors and may only be at a level of around 2 percent or 3 percent. So, advice to hay fever sufferers to stay indoors is clearly well founded.

Some significant air pollution is generated solely within buildings.  Under extreme adverse conditions, what is known as sick building syndrome can have detrimental effect on wellbeing and also reduce productivity in affected offices. 

In the domestic situation, the main internally-generated pollutants are:  tobacco smoke, radon, gas and particles from burning fuels, chemicals released from household products, as well as gases released from a number of building materials. 

For example, gaseous aldehydes are released from fabric softeners, ketones from dry-cleaning fluids and various waxes and polishes, alkanes will be released from solvents and other cleaning compounds, esters from glues and resins, and alcohols from spray can propellants.  

Toluene and xylene may be released from paints, printed materials and electronic equipment. It’s been found that paints can continue to out-gas for many months after application.  Formaldehyde out-gases from plywood, and flame-retardant chemicals from furniture and carpets.  Some computer printers generate ozone.  

Candles and incense may release particles as well as benzene and styrene. Carbon monoxide can be a problem with domestic gas cookers, and acrolein is a dangerous product of overheated cooking oils. Pesticides can be released from wood preservatives. Even some house-plants release their own terpenes.

Organic compounds

This wide range of exotic chemicals can cause damage to liver, kidneys and the nervous system and irritate the eyes, nose and throat as well as cause headaches, nausea and, in the extreme cases, be an agent of various cancers.  

Children, pregnant women, existing asthma sufferers, those with cardiovascular or respiratory diseases, and the elderly may all be more vulnerable.

The US Environment Protection Agency reports that, on average, the level of such volatile organic compounds (VOCs) is between 2 and 5 times higher inside houses compared to outdoors.  Some activities, for example paint stripping, can raise levels to 1000 times higher.

As a separate issue, there are numerous domestic biogenic air-borne pollutants that may cause heath impacts.  These include fungal spores, bacteria, viruses, dust mites, arthropods and protozoa deriving from pets, damp conditions, and poorly maintained systems like drip-trays and ventilation systems.  Allergies, infections and asthma attacks are potentially related symptoms.

Many of these in-house problems may be exacerbated by our response to climate change in trying to seal our homes better in order to provide improved energy efficiency, and by installing thermal insulation, some types of which may themselves create air pollution.  

Higher temperatures and higher humidity, related to the sealing of buildings and stagnation of air, may aggravate the formation of fungi and moulds and the release of spores.

Mitigating actions

There are mitigating actions that householders can take: opening windows to ventilate the house, particularly when the outside air has low pollution levels, installing and routinely using an extractor hood over the cooker, extracting damp air from bathrooms, ensuring that low VOC paints are used when decorating, using natural materials for carpets and other coverings whenever possible, and minimising the use of propellent sprays and man-made polishes.

Another proactive response might be to deploy pot plants in the house.  The plant-root-soil zone provides particularly effective bio-degradation of VOCs, while leaves also denature chemicals and physically trap air-borne particles.  

It’s important to select the right plants, among the best being aloe vera, several palms, rubber plant, dragon tree, ivy, ficus benjaminaand the spider plant – all of which are quite tough and thrive in indoor climates. 

Studies of the efficacy of plants/soils was carried out by NASA to investigate ways to purify air in the sealed confines of a space station.  Their experiments showed between 50 percent and 70 percent removal of formaldehyde and benzene and slightly lower, around 40 percent, removal of trichloroethylene over a 24-hour period in a sealed chamber.  

These plants also release very little VOCs, such as terpenes, themselves.  Real life experiments produce quite varying and sometimes contradictory results, but it has been suggested that one plant is required for every 10 sq m of floor area, perhaps occupying about 10% of overall air space.

Coordinated regulation

The World Health Organisation (WHO) has been publishing guidelines on safe concentrations of common indoor air pollutants since 1987.  There are, of course, legal requirements under Health & Safety legislation for employees to be provided with a safe working environment, but that doesn’t extend to the home environment.  

There are numerous efforts by many suppliers these days to provide low-emission products, so clearly there is a growing market of consumers who are aware of some of the claims of adverse health impacts of indoor air pollution.  

But there remains little in the way of standardisation or coordinated regulation or consistent labelling.

There can be competing priorities as well – for example, Trading Standards will enforce on retailers the requirements for low combustibility of domestic soft furnishings, but that will likely entail use of chemical fire retardants on the fabrics, which can then be released into indoor air and dust.  These retardants are well-known carcinogens and endocrine disruptors which also bio-accumulate in the body and persist for a very long time.

The most dangerous chemicals have fairly recently been banned and there is an ongoing debate about the relative levels of risk to human health of possible house fires compared to exposure to the chemicals.

Complex environment 

Since we spend so much time indoors, and much of that at home, it is perhaps anomalous, as WHO points out, that routine and standardised monitoring is undertaken throughout Europe for outdoor air quality but routine monitoring of indoor air quality hardly exists.  

The European Commission admits that the variability and complexity of the indoor environment makes risk assessment difficult and that, currently, the studies are insufficient.  

It is suggested that indoor air may contain as many as 900 different chemicals, particles, and bio-materials with potential health effects. The Commission is not in favour of enforceable indoor air quality standards, because of the privacy of the indoor space and also the impracticality of surveillance.  

It seems that a continuing effort to create an evidence base and then apply appropriate building regulations and product standards might be a more acceptable route.  

In the UK, it is really only regulations applying to paints that currently impose some control over the potential release of VOCs into indoor air, while UK Building Regulations also impose requirements on ventilation to limit levels of a small range of aggregated indoor pollutants, but based on an assumption of outdoor air being unpolluted.

Aggressive action

The new UK air quality strategy commits the Government to raise awareness specifically of VOC pollution indoors, to seek a voluntary labelling code for VOC-containing products, and to work with producers to reduce the content of harmful chemicals.  These are worthy but rather weak commitments.

No single Government Department has sole responsibility for indoor air pollution, so perhaps a more detailed, co-ordinating strategy and action plan for indoor air pollution in isolation is now necessary.

If the costs to the economy are nearly 2 percent of GDP, or about £30 Bn per year, then the problem surely merits much more aggressive action with regulatory controls on home design, construction materials, finishes, domestic equipment, furniture, fittings and consumables.

Overall, the task is huge and it may well be much better undertaken at European level, or even at wider international level.  

Failing that, there’s no excuse not to act now, here at home, and in the home.

This Author 

James Curran is the former chief executive of the Scottish Environment Protection Agency.  He researches and writes on sustainability, climate change, ecosystems, air pollution, and green finance.

Image: Ervins Strauhmanis, Flickr.