Figure 1: Combustion of fossil fuels is a major source of harmful fine particulates worldwide. Indoor burners, boilers, heaters and manufacturing appliances, together with the ingress of vehicular emissions from outside can give rise to PM10 and PM2.5 at harmful levels.
Figure 1: Combustion of fossil fuels is a major source of harmful fine particulates worldwide. Indoor burners, boilers, heaters and manufacturing appliances, together with the ingress of vehicular emissions from outside can give rise to PM10 and PM2.5 at harmful levels.
Figure 2: Mechanical air filtration is an effective particulate removal strategy. Even in sensitive environments, such as hospitals, high efficiency particulate air (HEPA) systems can reliably remove airborne pathogens.
Figure 2: Mechanical air filtration is an effective particulate removal strategy. Even in sensitive environments, such as hospitals, high efficiency particulate air (HEPA) systems can reliably remove airborne pathogens.
Figure 3: Activated carbon is a common general-use medium used in dry-gas scubbing for VOC, odour and other pollutant removal. (Image courtesy of Blücher)
Figure 3: Activated carbon is a common general-use medium used in dry-gas scubbing for VOC, odour and other pollutant removal. (Image courtesy of Blücher)

Introduction

For most people in the world the majority of their life is spent indoors. In industrialised countries across Europe, the USA and Canada and even “outdoor-lifestyle” Australia at least 90% of people’s time is estimated to be spent in the indoor environment. “Indoor” definitions are not consistent, but can be considered any place where at least one hour is occupied by people of varying health, including at home, in the office/workplace, school, hospital, and shops; occupation of these areas dominate daily life in the industrialised world.
Environmental and health protection of the atmospheric environment across the world has focused largely on the outdoor environment, including large-scale climate change considerations and localised urban air quality. Regulation of indoor air quality (IAQ) has developed in parts of the USA in its own right, but is often approached in an indirect way such as being implemented through workplace health & safety (H & S) regulation in combination with regional air quality plans. Across many countries, it is largely through ensuring H & S compliance for industrial and public indoor spaces that IAQ is assessed and improvement needs are recognised.

Sources of indoor air pollution

Indoor pollutants can be categorised usefully as being particulate or gaseous. These pollutants may be generated indoors or can ingress from the outdoor environment. On average, we breathe between 20 and 30 kg of air daily. This is significant relative to other forms of ingestion (food and drink being 3 – 4 kg daily). Although the human body naturally filters and expels much of the inhaled air without absorption of many components, our biologically protective measures are not entirely efficient and particles can enter, lodge and remain within the bronchial tree and alveolar sacs where toxic components, including suspected carcinogens, can be slowly absorbed whilst other irritant or allergenic health effects and the absorption of harmful gases (e.g. carbon monoxide, radon) into the blood-stream can adversely impact.
Particulate pollutants include those anthropogenically generated, such as cooking smokes, environmental tobacco smoke (ETS) and fossil-fuel combustion products (e.g. particulate carbon PM10 and PM2.5, Figure 1), natural dusts, and also a wide range of biological and microbiological sources such as dust mite wastes, pollen, spores, bacteria, viruses, etc. Clean air will contain about 108 particles/m3, whilst rural air, urban air and tobacco smoke zone containing about 109, 1011 and 1014 particles/m3 respectively. The health effects of particulates are as wide-ranging as the varied chemical compositions of the particles themselves. Asthma, a process whereby the bronchi spasm, constrict, and produce excess mucus, can be triggered by a number of particulate stimuli including ETS, pollen and dust mites. Carbonaceous particulates from diesel combustion and components in ETS are strongly linked with carcinogenic effect. Industrial air pollutants, including lead, mercury and asbestos all have associated World Health Organisation (WHO) guidelines in recognition of their toxicity.
Gaseous pollutants of IAQ concern also have varied anthropogenic and natural sources. In offices across much of the world various volatile organic compounds (VOCs) are prevalent, derived from treatments, paints and varnishes applied to soft furnishings and furniture. Some are irritants whilst others are potentially much more harmful, notably those based on aromatic and polyaromatic carbon (benzene) ring structures. Combustion products from commercial and industrial boilers and other appliances can generate carbon monoxide if burners are not properly maintained. Ambient (outdoor) air quality can also impact on the indoor environment. Important gaseous pollutants generated externally include sulphur dioxide and nitrous oxides (acidic irritants), aeolian transported pesticides and tropospheric ozone. Ozone is a secondary pollutant caused by the photo-catalytic reactivity of vehicular emissions; rural environments downwind of urban ozone sources can be severely impacted as certain ozone-destructive mechanisms based on NOx – cycling will not necessarily occur as intensively outside the urban environment. Of the naturally occurring gaseous IAQ pollutants, radon is probably most focused upon. Generated during the radioactive decay of naturally occurring uranium in rocks, radon gas can migrate into indoor spaces and build-up to potentially harmful levels unless adequate abatement or ventilation measures are used. Radon itself decays with a half-life of 3.8 days (emitting alpha-particles) and generates particulate daughter compounds (including polonium, bismuth and lead) which can be inhaled directly or whilst attached electrostatically to other indoor particulates.
Indoor air exchanges at a rate dependent upon infiltration (seepage of air through gaps, cracks and joints in buildings and floors) and natural and mechanical ventilation. Natural ventilation draughts through windows and doors are a function of pressure gradients caused by indoor and outdoor air temperature differences, and accordingly can vary temporally. Although ingress to the indoors of outdoor pollutants is an undesirable effect, the indoor environment requires a certain degree of exchange to avoid the build-up of pollutants generated in-situ. For industry, active air exchange enhancement may prove essential to prevent the build-up of harmful levels of gaseous or particulate contaminants in the presence of large quantities or high concentrations of manufacturing precursors or products.

Strategies to mitigate IAQ problems

Traditionally effective infiltration and natural ventilation has been used worldwide to effect a controlled exchange of indoor air to abate IAQ problems. However, in the past few years in the industrialised world conflicting energy efficiency considerations have resulted in an increase in building “tightness” which adversely impacts infiltration mechanisms and can discourage extensive use of natural ventilation. Concurrently, a greater understanding of potential health impacts caused by poor IAQ has been investigated by the scientific health community and policy makers have responded. One strategy to abate IAQ problems has been to minimise the sources of harmful substances where possible. For instance, fireretarding and stain-resistant formulations have been applied to furnishings for both the commercial and domestic market for many years. Awareness of the potential harm of many of these products has resulted in improved, lower emitting and less harmful formulations being introduced. Benzene, formaldehyde, phenolic, polyaromatic and chlorinated compounds within formulations are carefully controlled as their toxic or carcinogenic hazards have been scrutinised. Industrially, advances in health & safety atmospheric control including COSHH and similar hazard assessments, together with better availability of and access to material safety data sheets, have improved user understanding and workplace risk assessment with respect to air quality. In Europe, the recently introduced REACH regulations (Registration, Evaluation, Authorisation and Restriction of Chemicals, 2007) will ensure harmful chemicals included in manufactured products and formulations brought in from outside the region are also regulated. Appliance combustion efficiencies are routinely covered by legislation to limit human exposure to carbon monoxide.
Some countries have detailed IAQ objectives such as period-defined maximum allowable concentration for some the most harmful substances, often adopting or exceeding WHO guidelines, although the method of legal application varies. For example, IAQ in the USA relies on initiatives from individual states (e.g. Washington, California) using the United States Environmental Protection Agency (USEPA) and other guideline limits, whilst Australia have adopted National Health and Medical Research (1993) limits, and in Canada residential occupants are protected by Health & Welfare (1987). Elsewhere, in the UK, Local Air Quality objectives and health & safety legislation form major IAQ enforcement routes.
Ingress of poor quality outside air is clearly undesirable. In Europe for example, much attention has focused on local air quality in urban areas. In the UK local authorities are responsible for identifying areas with unacceptable air quality (based primarily on ambient NOx, ozone and PM10 concentrations) and implement action plans in defined zones which may include improved traffic management to reduce vehicular emissions. It is not uncommon for the worst local air quality problems to be around hospitals, shopping areas or public transport hubs where large numbers of vehicles move slowly or idle whilst stationary. Geological mapping has been widely used to identify “at risk” areas for radon. Government schemes allow widespread ambient radiation testing across “risk” regions, and where trigger concentrations are indicated householders can arrange for detailed radon assessment and install ventilation systems to abate the problem. Different governments’ assessment of safe IAQ is demonstrated by the differing radon limits set by the USA, Canada and Australia (4 pCi/L (ca. 150 Bq/m3), 200 Bq/m3 and 800 Bq/m3 respectively).
Pollutant source control, user-awareness, local air quality controls and health & safety legislation are important strategies in controlling IAQ across many countries. Where problems still exist, improved mechanical ventilation is often the solution and specific air quality problems can be reduced through installing active air treatment within in heating, ventilation and air conditioning (HVAC) systems. The importance of maintaining good-quality indoor air is recognised particularly in commercial and educational sectors (to minimise sickness absence and maintain productivity) and in the medical care sector (for example, to combat airborne infection).
Particle removal can be achieved through mechanical air filtration (including the range of HEPA, high efficiency particulate air filters, Figure 2) or by means of electronic air cleaners (including electrostatic precipitators). Filtration is achieved through a combination of physical mechanisms including straining, impaction, interception and diffusion. The process can be enhanced by utilising electrostatic separation by the incorporation of permanently static charged plastic materials in filters.
Electronic air cleaners operate differently by applying high voltages to statically charge dust which is then attracted to oppositely charged plates on the cleaner. Such air-cleaners can, however, generate ozone and airborne charged ultra-fine particles which have their own IAQ concerns, so their applications need to be carefully considered.
Gas (including odour) removal is not generally highly effective using standard particle filtration devices. Although some manufacturers have successfully developed layered filter-systems incorporating activated carbon to concurrently remove gases with some success, separate gas filtration systems will generally improve effectiveness in sensitive applications. A range of thermal (ceramic) and PTFE/polypropylene gas filters is commercially available for specialised industrial applications such as chemical storage tank vent filtration and pathogen removal (legionella, etc.) applications.
Pathogen destruction can be achieved using UV lamps, with UV germicidal irradiation (UVGI) forming a supplementary option for filtration. However, it is possible that irradiation time-doses may not be sufficient in many cases for the necessary effectiveness. A promising approach to IAQ improvement is organismal and VOC destruction through photocatalytic oxidation (PCO). Here, pollutants are adsorbed to a surface containing a titanium dioxide catalyst (possibly modified, for example with tungsten oxide). UV-C light activates the catalyst which breaks organic pollutants down, nominally into carbon dioxide and water. Whilst the process undeniably is effective on a macro-scale, some concerns remain as to whether PCO generates small quantities of undesirable by-products (e.g. formaldehyde) and the USEPA suggests PCO use domestically may not treat the full range of indoor pollutants, although manufacturers are confident of their effectiveness and safety.

Filtration systems for IAQ

Mechanical filters come in a range of forms. Simple flat (panel) filters do not generally improve IAQ but serve to protect HVAC equipment. However, IAQ-effective filtration is achieved through more efficient pleated filters and extended surface filters (which increase the depth and surface area of the filter), whilst electrostatically enhanced systems offer high performances in particle filtration. Pleated filters are generally manufactured from synthetic or cotton/polyester blended fibres and, depending upon specific design, typically achieve MERV ratings in the range 6 to 13, whilst higher efficiency mechanical filters achieve MERV 14-16.
For the higher efficiency particle filters, alternative proposed rating systems to MERV can prove useful for discriminating performance. The atmospheric-dust spot test can be useful in determining the capture of particles as small as 0.3 μm. Under this test, many panel systems rate <5%, whilst a 40% efficiency rated system will capture pollen and dust well and a 90%-rated system is effective for ETS and combustion product removal. For systems exceeding 98% efficiency on this test, further comparative refinement is possible using the finer medium DOP-smoke penetration test.
Though effective at removing particles on passage through the filter (better than 80% of <2.5 μm fine particles for certain higher efficiency filters), IAQ improvements after particle filtration can still be limited as large particles tend to settle quickly within the indoor environment. These do not necessarily reach filter systems and can be repeatedly disturbed and dispersed into the indoor atmosphere. Furthermore, filter efficiency over time can vary. Many filters improve in capability upon use as entrapped material reduces porosity and the capture of fine particles can improve with age to a certain extent. In contrast, the effectiveness of electrostatically enhanced filters diminishes with time, as charged filter surfaces become neutralised by attached counter-charged particles. For example, in studies carried out by the EU (REC06, air filters for better IAQ), tested filters dropped from 80% to 20% removal efficiency after just a few weeks in operation.
The excessive build-up of filtered material is undesirable, since associated pressure drops across filters can impair effectiveness. The energy consumption of operating filters is a major cost in the overall life-cycle analysis (typically 60–80% of the energy use including manufacture, maintenance, use and disposal), an important factor when energy prices are high. Excessive pressure loss can lead to large increases in energy consumption, where energy used (kWh) = (air flow (m3/h) x operation time (h) x mean pressure loss (Pa))/(1000 x fan efficiency (%)).
HEPA systems (MERV 17-20) may be required in specific circumstances. Hospitals (and other locations with sensitive occupants such as children) may opt for HEPA systems to minimise the chance of pathogen transmission, whilst some specialised industries (e.g. nuclear) may need particularly effective emission abatement. HEPA systems often use specialised filter materials. Patented minipleat designs offer exceptionally high surface areas, whilst other designs can incorporate both high-quality (>99.9% or better of 0.12 μm particles is achievable) particle removal together with specific gas treatment technologies, and manufacturers continue to develop a wide range of advanced filter materials. Other manufacturers produce biodegradable (e.g. polylactyl) material designs for air filtration based around PPE equipment (face-masks, etc.).
Gas filtration can be effective in odour control and the removal of important irritants from indoor air. Most development in this sector has focused on dry-filtration through adsorptive media (Figure 3). Activated carbon media is effective for general purpose treatment, including sulphidic compounds, many of which (hydrogen sulphide, mercaptans, etc.) cause odour problems for a variety of industries, whilst other advanced materials or blended sorbents can be more effective for VOC, ammonia or oxidative treatment (e.g. permanganate impregnated alumina). Most media permanently adsorb pollutants, so a sufficiently slow air flow and humidity controls may be necessary to avoid gas break-through, whilst such media have a finite lifetime before becoming spent. Other media have been developed which chemically react with pollutants to generate less harmful products.

Conclusions

Most of us spend most of our time indoors in homes, schools, the workplace, shopping malls or hospitals. The air we breathe can be degraded by a wide variety of contaminants, natural, synthetic, biological and inorganic. Governments across the world have begun to address IAQ problems through reduced use of harmful pollutants, improvement strategies for outdoor air quality, and improved recognition of radon-affected areas. Legislative controls are implemented in a variety of ways, both direct and indirect.
Energy efficiency measures have tended to improve building “tightness” and reduce air exchange via infiltration and natural ventilation. Therefore, mechanical ventilation has been widely adopted to avoid harmful IAQ through pollutant build-up. Filtration of the air can remove both particles and gases, though few systems can do both effectively at the same time.
Particle filters will improve IAQ only if they are of pleated or extended-area design, whilst the use of advanced media including electrostatic enhancement will improve filtration to potentially a very degree (MERV 16) even for non-HEPA designs. Against filtration efficiency, energy costs, maintenance requirements, installation logistics and system noise need to be considered in choosing the best system for an application. Although other methods of particle destruction are being developed and available, including UVGI and PCO, air filtration is likely to remain the method of choice for IAQ improvement in future years. Gas filtration, primarily using dry-scrubbing through granulated media is optimally achieved through separate treatment systems.
Contact:
Anthony Bennett is Technical Director at Clarity Authoring.
Contact via www.clarityauthoring.com.