Indoor Air Pollution

By Royal Society of Chemistry

Time–activity diaries kept by members of the general public indicate that on average people spend around 90% of their time indoors. This is associated with considerable exposure to air pollutants as not only is there infiltration of pollutants from outdoors, there are also emissions indoors that can lead to elevated pollutant concentrations. Despite this, and the fact that the WHO produces air quality guidelines for indoor air, the only statutory requirements for monitoring of airborne pollutant concentrations relate to the outdoor environment. Given its importance as a source of air pollution exposure, increasing attention is being given to pollution of the indoor environment.

This volume considers both chemical and biological pollutants in the indoor atmosphere from their sources to chemical and physical transformations, human exposure and potential effects on human health. It is a valuable reference for those working in in environmental policy, civil and environmental engineering as well as for atmospheric chemists.

• Language: English
• Publisher: Royal Society of Chemistry
• Release date: May 8, 2019
• ISBN: 9781788018036

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Indoor Air Pollution

ISSUES IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY

SERIES EDITORS:

R. E. Hester, University of York, UK

R. M. Harrison, University of Birmingham, UK

EDITORIAL ADVISORY BOARD:

S. J. de Mora, Plymouth Marine Laboratory, UK, G. Eduljee, SITA, UK, Z. Fleming, University of Leicester, UK, L. Heathwaite, Lancaster University, UK, S. Holgate, University of Southampton, UK, P. K. Hopke, Clarkson University, USA, P. S. Liss, University of East Anglia, UK, S. Pollard, Cranfield University, UK, X. Querol, Consejo Superior de Investigaciones Científicas, Spain, D. Taylor, WCA Environmental Ltd, UK, N. Voulvoulis, Imperial College London, UK.

TITLES IN THE SERIES:

  1: Mining and its Environmental Impact

  2: Waste Incineration and the Environment

  3: Waste Treatment and Disposal

  4: Volatile Organic Compounds in the Atmosphere

  5: Agricultural Chemicals and the Environment

  6: Chlorinated Organic Micropollutants

  7: Contaminated Land and its Reclamation

  8: Air Quality Management

  9: Risk Assessment and Risk Management

10: Air Pollution and Health

11: Environmental Impact of Power Generation

12: Endocrine Disrupting Chemicals

13: Chemistry in the Marine Environment

14: Causes and Environmental Implications of Increased UV-B Radiation

15: Food Safety and Food Quality

16: Assessment and Reclamation of Contaminated Land

17: Global Environmental Change

18: Environmental and Health Impact of Solid Waste Management Activities

19: Sustainability and Environmental Impact of Renewable Energy Sources

20: Transport and the Environment

21: Sustainability in Agriculture

22: Chemicals in the Environment: Assessing and Managing Risk

23: Alternatives to Animal Testing

24: Nanotechnology

25: Biodiversity Under Threat

26: Environmental Forensics

27: Electronic Waste Management

28: Air Quality in Urban Environments

29: Carbon Capture

30: Ecosystem Services

31: Sustainable Water

32: Nuclear Power and the Environment

33: Marine Pollution and Human Health

34: Environmental Impacts of Modern Agriculture

35: Soils and Food Security

36: Chemical Alternatives Assessments

37: Waste as a Resource

38: Geoengineering of the Climate System

39: Fracking

40: Still Only One Earth: Progress in the 40 Years Since the First UN Conference on the Environment

41: Pharmaceuticals in the Environment

42: Airborne Particulate Matter

43: Agricultural Chemicals and the Environment: Issues and Potential Solutions, 2nd Edition

44: Environmental Impacts of Road Vehicles: Past, Present and Future

45: Coal in the 21st Century: Energy Needs, Chemicals and Environmental Controls

46: Energy Storage Options and Their Environmental Impact

47: Plastics and the Environment

48: Indoor Air Pollution

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ISSUES IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY

EDITORS: R.M. HARRISON AND R.E. HESTER

48

Indoor Air Pollution

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Issues in Environmental Science and Technology No. 48

Print ISBN: 978-1-78801-514-1

PDF ISBN: 978-1-78801-617-9

EPUB ISBN: 978-1-78801-803-6

Print ISSN: 1350-7583

Electronic ISSN: 1465-1874

A catalogue record for this book is available from the British Library

© The Royal Society of Chemistry 2019

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Apart from fair dealing for the purposes of research for non-commercial purposes or for private study, criticism or review, as permitted under the Copyright, Designs and Patents Act 1988 and the Copyright and Related Rights Regulations 2003, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry or the copyright owner, or in the case of reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page.

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Preface

It is an often quoted and highly significant statistic that in the developed world the average person spends about 90% of their time indoors. Despite this, almost all of our knowledge of the effects of ambient air pollution derives from outdoor measurements and air quality management policy is focused upon monitoring and controlling levels of pollution outdoors. Slowly, however, things are changing and the scientific study of pollution of the indoor environment is increasing, although there still have been very few studies of the effects of indoor pollutant exposures on health. In the less developed world, such exposures can be extreme when they arise from unvented combustion appliances used in the home. However, the focus of this volume is upon the developed world.

Indoor air pollution arises both from indoor emissions and from the infiltration of outdoor air. The first two chapters by Ioar Rivas and co-authors and Otto Hänninen and Patrick Goodman look, respectively, at the indoor and outdoor sources that affect the quality of air in the indoor environment. There are many human activities that lead to pollution of the indoor environment and, among other things, the first chapter looks closely at the school as an indoor environment that is likely to experience high pollution levels. The way in which outdoor air pollutants enter the indoor environment is considered in depth in the second chapter. The third chapter, by Tuan Vu and Roy Harrison, considers the chemical and physical properties of pollutants in the indoor environment. Factors such as the size distribution of particulate pollutants and their volatility determine their lifetime in the indoor environment, in addition to influencing the dose to the lung when the indoor air is inhaled.

For pollutants that have major indoor sources but are relatively scarce in the outdoor environment, passage from indoors to outdoors can be a significant source of pollutants in the outdoor environment. In the fourth chapter, Stuart Harrad uses halogenated chemicals, such as polychlorinated biphenyls and brominated flame retardants, as a case study of indoor emissions as a source of outdoor pollution. Chemical reactions in the indoor atmosphere can also be important, both as a source of newly formed pollutants and as a cleansing mechanism for some toxic molecules. It has recently been realized that the fragrances used in many domestic products, designed to produce a pleasant smell in the indoor atmosphere, can be oxidized to form particles of potentially high toxicity. This and other indoor chemical processes are reviewed by Nicola Carslaw in the fifth chapter. Many of the reactions important in the outdoor atmosphere are also found to affect indoor air.

Up to this point, this volume has considered indoor pollution as primarily a chemical phenomenon. However, it should not be forgotten that the indoor atmosphere contains many biological particles, and this topic is addressed in the sixth chapter by Ian Colbeck and Corinne Whitby. Modern methods of molecular biology are facilitating the characterization of organisms in the indoor atmosphere; these can arise from a range of human activities, including poor building maintenance, and some can present a significant threat to health, particularly through their allergenic properties. In the seventh chapter, Juana Maria Delgado-Saborit takes a comparative look at indoor and outdoor air as contributors overall to air pollution exposure; this serves to highlight the importance of the indoor atmosphere as an exposure medium. Such exposures can lead to adverse effects on health, and in the final chapter Robert Maynard gives a succinct overview of this vast area of research and highlights some of the adverse impacts of pollutant exposure occurring in the indoor environment.

We are delighted to have attracted this group of leading researchers to provide authoritative overviews of specific topic areas and we believe that overall the volume represents a comprehensive evaluation of many of the scientific characteristics and implications of indoor air pollution. This volume will prove valuable to scientists, students, consultants and policymakers seeking definitive insights into the topic of indoor air pollution.

Ronald E. Hester

Roy M. Harrison

Contents

Editors

List of Contributors

Indoor Sources of Air Pollutants

Ioar Rivas, Julia C. Fussell, Frank J. Kelly and Xavier Querol

1Introduction

2Indoor Sources in Homes

2.1 Sleeping

2.2 Cooking

2.3 Cleaning

2.4 Heating

2.5 Tobacco Smoking

2.6 Human Occupancy

2.7 Building and Furniture Materials

3Indoor Sources in Offices and Schools

3.1 Human Occupancy and Other Determinants of Indoor School Air Quality

3.2 Computers, Printers and Photocopiers

4Indoor Sources in Other Microenvironments

4.1 Restaurants

4.2 Hair Salons

4.3 Nail Salons

4.4 Fitness Centres

5Final Remarks

Abbreviations

References

Outdoor Air as a Source of Indoor Pollution

Otto Hänninen and Patrick Goodman

1Introduction

2Outdoor Air Pollutants

3Infiltration of Outdoor Air Pollution

3.1 Overview of Physical and Chemical Processes

3.2 Observed Infiltration of Some Health-relevant Gases

3.3 Observed Infiltration Levels of Particles

3.4 Impacts of Aerosol Size Distribution

4Epidemiological Evidence

5Outdoor Air Intake and Ventilation

5.1 Ventilation Standards and CO 2 Levels

5.2 Health-based Ventilation Guidelines

5.3 Filtration of Air in Mechanical Ventilation Systems

6Special Cases

6.1 Radon from Soil

6.2 Second-hand Smoke from Outdoors

7Conclusion

Nomenclature

Acknowledgements

References

Chemical and Physical Properties of Indoor Aerosols

Tuan V. Vu and Roy M. Harrison

1Introduction

2Aerosol Dynamics

2.1 Deposition

2.2 Coagulation

2.3 Gas/Particle Partition: Evaporation and Condensation

2.4 Particle Formation in Indoor Environments

3Physicochemical Properties of Indoor Aerosols

3.1 Physicochemical Properties of Aerosols Emitted from Five Major Indoor Sources

3.2 Implications for the Lung Dose

4Closing Thoughts

Acknowledgements

References

Indoor Emissions as a Source of Outdoor Pollution

Stuart Harrad

1Introduction

2Emission Sources and Rates of HSVOCs to Indoor Environments

3Indoor Versus Outdoor Concentrations of HSVOCs

4Influence of Indoor Contamination on Outdoor Concentrations

5What Are the Implications of Indoor Emissions Contributing to Outdoor Concentrations?

6Conclusion

References

Chemical Reactions in the Indoor Atmosphere

Nicola Carslaw

1Introduction

2Reactions in Indoor Air

2.1 Differences from Outdoor Air Chemistry

2.2 Key Reaction Types

3Reactions on Surfaces

3.1 On Materials

3.2 On People

4Conclusion

Acknowledgements

References

Biological Particles in the Indoor Environment

Ian Colbeck and Corinne Whitby

1Introduction

2Indoor Sources

3Air Sampling Methods

4Culturing and Limitations of Culture-based Techniques

5Sampling for Culture-independent Analysis of Bioaerosols

5.1 Cultivation-independent Methods for Quantifying Microbes in Bioaerosols

5.2 Polymerase Chain Reaction (PCR) and Quantitative Polymerase Chain Reaction (qPCR)

5.3 Microarrays

5.4 Next-generation Sequencing (NGS) to Characterize Bioaerosol Microbial Diversity

6Overview Of Indoor Concentrations

6.1 Size Distributions

7Guideline Values

8Conclusion

8.1 Perspectives

References

Indoor Air as a Contributor to Air Pollution Exposure

Juana Maria Delgado-Saborit

1Introduction

2Methodological Approaches

2.1 Equation for the Contribution of Individual Microenvironments to Personal Exposure to Pollutants

2.2 Characterization of Microenvironmental Concentrations

2.3 Determination of Time–Activity Patterns

2.4 From Exposure to Lung Dose: Contribution of Indoor Environments

3Factors Affecting the Contribution of Indoor Environments to Personal Exposure

3.1 Time–Activity Patterns

3.2 Factors Affecting Microenvironment Concentrations

4Contribution of Indoor Microenvironments to VOC Exposures

5Contribution of Indoor Microenvironments to NO 2 Exposures

6Contribution of Indoor Microenvironments to PM 2.5 and PM 10 Exposures

7Contribution of Indoor Microenvironments to BC Exposures

8Contribution of Indoor Microenvironments to UFP Exposures

9Contribution of Indoor Microenvironments to Exposures in Developing Countries

10 Contribution of Indoor Microenvironments to Exposures in Sensitive Populations

11 Conclusion

Acknowledgements

References

Health Effects of Indoor Air Pollution

Robert L. Maynard

1Linearity and Non-linearity of the Relationship Between Exposure Concentration and Risk: Particulate Matter

2Efficiency of Indoor Sources of Pollutants in Terms of Emission to Exposure (or Dose) Ratio

3Indoor Air Pollution in Developed Countries

4Air Pollutants of the Indoor Environment

4.1 Carbon Monoxide

4.2 Particulate Matter

4.3 Nitrogen Dioxide

4.4 Carcinogenic Indoor Air Pollutants

5Statics and Dynamics of Interventions to Reduce Household Air Pollution

References

Subject Index

Editors

Ronald E. Hester, BSc, DSc (London), PhD (Cornell), FRSC, CChem

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Ronald E. Hester is now Emeritus Professor of Chemistry in the University of York. He was for short periods a research fellow in Cambridge and an assistant professor at Cornell before being appointed to a lectureship in chemistry in York in 1965. He was a full professor in York from 1983 to 2001. His more than 300 publications are mainly in the area of vibrational spectroscopy, latterly focusing on time-resolved studies of photoreaction intermediates and on biomolecular systems in solution. He is active in environmental chemistry and is a founder member and former chairman of the Environment Group of the Royal Society of Chemistry and editor of ‘Industry and the Environment in Perspective’ (RSC, 1983) and ‘Understanding Our Environment’ (RSC, 1986). As a member of the Council of the UK Science and Engineering Research Council and several of its sub-committees, panels and boards, he has been heavily involved in national science policy and administration. He was, from 1991 to 1993, a member of the UK Department of the Environment Advisory Committee on Hazardous Substances and from 1995 to 2000 was a member of the Publications and Information Board of the Royal Society of Chemistry.

Roy M. Harrison, OBE, FRS, BSc, PhD, DSc (Birmingham), FRSC, CChem, FRMetS, Hon FFPH, Hon FFOM, Hon MCIEH

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Roy M. Harrison is Queen Elizabeth II Birmingham Centenary Professor of Environmental Health in the University of Birmingham. He was previously Lecturer in Environmental Sciences at the University of Lancaster and Reader and Director of the Institute of Aerosol Science at the University of Essex. His more than 500 publications are mainly in the field of environmental chemistry, although his current work includes studies of human health impacts of atmospheric pollutants as well as research into the chemistry of pollution phenomena. He is a past Chairman of the Environment Group of the Royal Society of Chemistry for whom he edited ‘Pollution: Causes, Effects and Control’ (RSC, 1983; Fifth Edition 2014). He has also edited An Introduction to Pollution Science, RSC, 2006 and Principles of Environmental Chemistry, RSC, 2007. He has a close interest in scientific and policy aspects of air pollution, having been Chairman of the Department of Environment Quality of Urban Air Review Group and the DETR Atmospheric Particles Expert Group. He is currently a member of the DEFRA Air Quality Expert Group, the Department of Health Committee on the Medical Effects of Air Pollutants, and Committee on Toxicity.

List of Contributors

Nicola Carslaw, Department of Environment and Geography, University of York, York YO10 5DD, UK. Email: [e-mail]

Ian Colbeck, School of Biological Sciences, University of Essex, Colchester CO4 3SQ, UK. Email: [e-mail]

Juana Maria Delgado-Saborit, ISGlobal Barcelona Institute for Global Health – Campus MAR, Barcelona Biomedical Research Park (PRBB), c/ Doctor Aiguader 88, 08003 Barcelona, Spain. Email: [e-mail]

Julia C. Fussell, Department of Analytical, Environmental and Forensic Sciences, School of Population Health and Environmental Sciences, Franklin-Wilkins Building, King's College London, 150 Stamford Street, London SE1 9NH, UK

Patrick Goodman, Dublin Institute of Technology, Dublin, Ireland

Otto Hänninen, National Institute for Health and Welfare, 70701 Kuopio, Finland. Email: [e-mail]

Stuart Harrad, School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Email: [e-mail]

Roy M. Harrison, Department of Environmental Health and Risk Management, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK, and Department of Environmental Science/Center of Excellence in Environmental Studies, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia. Email: [e-mail]

Frank J. Kelly, Department of Analytical, Environmental and Forensic Sciences, School of Population Health and Environmental Sciences, Franklin-Wilkins Building, King's College London, 150 Stamford Street, London SE1 9NH, UK

Robert L. Maynard, Honorary Professor, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Email: [e-mail]

Xavier Querol, Institute of Environmental Assessment and Water Research, IDAEA-CSIC, c/ Jordi Girona 18–26, 08034 Barcelona, Spain

Ioar Rivas, Department of Analytical, Environmental and Forensic Sciences, School of Population Health and Environmental Sciences, Franklin-Wilkins Building, King's College London, 150 Stamford Street, London SE1 9NH, UK. Email: [e-mail]

Tuan V. Vu, Department of Environmental Health and Risk Management, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Corinne Whitby, School of Biological Sciences, University of Essex, Colchester CO4 3SQ, UK

Indoor Sources of Air Pollutants

Ioar Rivas,* Julia C. Fussell, Frank J. Kelly and Xavier Querol

*Corresponding author.

ABSTRACT

People spend an average of 90% of their time in indoor environments. There is a long list of indoor sources that can contribute to increased pollutant concentrations, some of them related to human activities (e.g. people's movement, cooking, cleaning, smoking), but also to surface chemistry reactions with human skin and building and furniture surfaces. The result of all these emissions is a heterogeneous cocktail of pollutants with varying degrees of toxicity, which makes indoor air quality a complex system. Good characterization of the sources that affect indoor air pollution levels is of major importance for quantifying (and reducing) the associated health risks. This chapter reviews some of the more significant indoor sources that can be found in the most common non-occupational indoor environments.

1Introduction

Outdoor air pollution has been extensively studied for a considerable amount of time, with the first long-term fixed outdoor monitoring stations being established in the 1960s in the UK.¹ Traditionally, outdoor concentrations have been used in epidemiological studies to evaluate health effects of air pollution.²,³ However, people spend an average of 90% of their time indoors⁴,⁵ and, therefore, indoor environments will contribute significantly to the total daily exposure, as the latter is the product of the pollutant concentration (which will vary with time and with space/microenvironment) and the time that a person is in contact with the given pollutant.⁶

Although the indoor environment has been explored much less extensively than the outdoor atmosphere, there has been a focus on characterizing indoor air pollution over the past few decades, and several studies have attempted to understand the sources and the various parameters that impact on indoor air quality (IAQ). However, there are still major challenges to be addressed. It is difficult to characterize air pollution in indoor environments, for two main reasons in particular: (1) people spend time in a wide range of indoor environments (e.g. home, office, school, restaurants) and, therefore, are exposed to a wide range of indoor sources of air pollution; home is the indoor environment where people spend most of their time (58–69%), followed by their work environment (28%) (Table 1); (2) real-world measurements of indoor environments require consent and willingness of the owner or the person responsible for the facility to participate in a study, and these may sometimes be difficult to obtain. Air pollution monitoring instruments are usually large and noisy, hence space requirements and disruption may cause unbearable annoyance. Furthermore, high instrumentation costs may limit the monitoring to a single room or area of the building. In recent years, this issue has been overcome by the use of miniaturized instruments or sensors,⁷ although sometimes their use implies a trade-off in precision and/or accuracy and also in the air pollutants that can be measured [e.g. it is difficult to measure the chemical composition of particulate matter (PM) with a sensor].

Table 1 Percentage of time spent at home and in other microenvironments.

IAQ is affected by a series of sources, including outdoor particulate and gaseous pollutants that infiltrate indoors⁸ and PM and gases emitted indoors either by human activities (e.g. cooking, cleaning⁹,¹⁰) or by indoor materials, building surfaces¹¹ and biological surfaces.¹² Indoor activities are often characterized by short events, but with very high pollutant concentrations. A common source in most (probably all) environments is the infiltration of outdoor sources, which can explain a varying range as large as 30–80% of the indoor concentrations of PM2.5 (PM with a diameter of <2.5 μm),¹³ depending on many factors such as the building envelope and ventilation settings. In situations with the absence of or very few indoor sources (e.g. in homes during sleeping time), indoor concentrations of air pollutants (both gases and PM) usually show similar patterns to outdoor concentrations.¹⁴,¹⁵ The contributions of outdoor sources to indoor air pollution are covered in a later chapter. Moreover, indoor chemistry is determined by indoor conditions that differ from those outdoors such as sunlight incidence, temperature variability, dispersion and surface-to-volume ratio.¹⁶

Owing to this wealth of sources and chemical reactions, the composition, and thus toxicity, of indoor particles and gaseous compounds are dynamic and very complex.⁶ Various studies have found associations between exposure to indoor air pollution and impaired health. Poor IAQ in non-occupational settings has been associated with several negative health effects, including asthma exacerbation,¹⁷ increased blood pressure¹⁸ and ‘sick building’ syndrome, a set of non-specific symptoms (e.g. headaches, allergy, eye irritation) related to the time spent indoors and that may be partly explained by exposure to indoor air pollutants. In low- and middle-income countries, household air pollution (HAP) from the use of solid fuels in inefficient stoves for cooking or heating is a major issue. According to the Global Burden of Disease study, HAP is the tenth leading global risk factor for deaths [eighth for disability-adjusted life-years (DALYs)] and was responsible for 2.8 million deaths (and 85.6 million DALYs) in 2015.¹⁹

In contrast to ambient air pollutants, only a few countries (e.g. China, Portugal and Taiwan) have established standards for indoor air pollutants. Therefore, IAQ is not routinely monitored. However, the World Health Organization (WHO) has proposed some guidelines for some indoor air pollutants²⁰ (Table 2).

Table 2 Indoor air pollution guidelines.

Asbestos and radon and its decay products are often the targets of studies aimed at characterizing IAQ. However, asbestos fibres and radioactive particles are not discussed in this chapter, and readers are directed to many comprehensive publications on this topic such as the IARC Monograph on asbestos²¹ and the WHO Handbook on Indoor Radon.²²

2Indoor Sources in Homes

One may expect indoor concentrations of air pollutants to be lower than outdoors as the building may exert some kind of protection against outdoor sources of air pollution (e.g. traffic and industrial emissions). However, in addition to possible strong infiltration of some outdoor air pollutants,¹³ there are several routine domestic activities, such as smoking, cooking and cleaning, that constitute important sources of indoor pollution.

2.1 Sleeping

Of all our lifetime activities, sleeping occupies the most time. Time–activity pattern studies and surveys report similar trends in sleeping around the world, with about 8–9 h per day of sleeping on average across all ages.²³–²⁵ The extent of the time spent sleeping makes the bedroom an important microenvironment to explore.²⁶ Generally, most studies show that indoor air particle number concentrations (PNCs) of ultrafine particles (UFPs; particle size <100 nm) and black carbon (BC) reach their minimum in homes during unoccupied periods and during the night, when few active human activities occur.²⁷,²⁸

In addition to background concentrations of air pollutants from outdoor infiltration or generated previously from indoor activities, the bedroom will contain emissions from furniture and building materials. Of particular interest are soft furnishings such as mattresses and pillows, with which people are in close contact during their sleep. Mattress dust is comprised of a wide range of viruses, organisms (bacteria, fungi) and their allergens and inorganic dust,²⁶ all of which can be resuspended during movement. Moreover, a mattress is a source of a variety of (semi-)volatile organic compounds [(S)VOCs], such as plasticizers and flame retardants, which may volatilize and are known to be endocrine-disrupting chemicals.²⁹,³⁰ Little literature is available on human-induced particle resuspension from pillows, mattresses and other bedding items, but it has been reported to be comparable in magnitude to resuspension induced by other human indoor activities, such as walking.³¹

Bedrooms may sometimes be characterized by lower ventilation rates than the other home environments,³² which may hinder the dispersion of indoor-generated pollution and, therefore, favour its accumulation. Few studies are available on investigations of ventilation patterns and the presence of different air pollutants during time spent sleeping,³³ with inconclusive results. Owing to the time spent in this microenvironment, further research is needed to characterize exposure fully.

2.2 Cooking

Cooking activities have also been linked to increased concentrations of PM (and specially PNCs) and gaseous pollutants in indoor home environments. Studies have reported a wide variability between regions, owing to different stove types, fuels, cooking styles and food types.⁹,³⁴

2.2.1 Type of Fuel

Cooking emissions are of particular importance in developing countries, where populations rely strongly on solid fuels and use inefficient cookstoves (especially in rural areas).³⁵ Around half of the world's households are dependent on solid fuels (e.g. wood, crop wastes, animal dung and coal) for cooking.³⁶ The proportion varies across regions, with solid fuels being used in >60% of the households in Africa and Southeast Asia, 46% in the Western Pacific region, 35% in the Eastern Mediterranean area and much less (<20%) in the American continent and Europe.³⁶ Globally, the exposure to emissions from burning solid fuels (for cooking, heating and lighting) was responsible for 2.8 million deaths and 85.6 million DALYs in 2015.¹⁹ Several studies have linked the exposure to these emissions with adverse pregnancy outcomes (e.g. low weight at birth, stillbirths),³⁷ respiratory diseases (e.g. respiratory tract infection, including tuberculosis, and also aggravation of inflammatory lung conditions such as asthma), cancer,³⁸,³⁹ cardiovascular disease (e.g. stroke), and other health issues (e.g. eye diseases, skin ageing).⁴⁰,⁴¹

Several studies (most of them carried out in India and China) have reported that the person cooking in these regions (mainly women⁴²) are exposed to very high concentrations of cooking fumes/HAP. Fuel type has a significant influence on IAQ and subsequent health effects. Indeed, the significant impact of biomass and coal combustion on indoor air pollutant concentrations has been outlined in several publications.⁴³,⁴⁴ The hierarchy of fuels in order of decreasing PM2.5 concentrations reported in kitchens is the following: biomass, coal, kerosene and LPG/electric stoves,⁴⁵–⁴⁸ with biomass and coal having much higher emissions of PM2.5 than kerosene and LPG/electric. For instance, in a study in southern India, concentrations of respirable PM (PM with an aerodynamic diameter ≤4 µm; PM4) in households using biomass