There is growing evidence that projected climate change has the potential to significantly affect public health. In the UK, much of this impact is likely to arise by amplifying existing risks related to heat exposure, flooding, and chemical and biological contamination in buildings. Identifying the health effects of climate change on the indoor environment, and risks and opportunities related to climate change adaptation and mitigation, can help protect public health. We explored a range of health risks in the domestic indoor environment related to climate change, as well as the potential health benefits and unintended harmful effects of climate change mitigation and adaptation policies in the UK housing sector. We reviewed relevant scientific literature, focusing on housing-related health effects in the UK likely to arise through either direct or indirect mechanisms of climate change or mitigation and adaptation measures in the built environment. We considered the following categories of effect: (i) indoor temperatures, (ii) indoor air quality, (iii) indoor allergens and infections, and (iv) flood damage and water contamination. Climate change may exacerbate health risks and inequalities across these categories and in a variety of ways, if adequate adaptation measures are not taken. Certain changes to the indoor environment can affect indoor air quality or promote the growth and propagation of pathogenic organisms. Measures aimed at reducing greenhouse gas emissions have the potential for ancillary public health benefits including reductions in health burdens related heat and cold, indoor exposure to air pollution derived from outdoor sources, and mould growth. However, increasing airtightness of dwellings in pursuit of energy efficiency could also have negative effects by increasing concentrations of pollutants (such as PM2.5, CO and radon) derived from indoor or ground sources, and biological contamination. These effects can largely be ameliorated by mechanical ventilation with heat recovery (MVHR) and air filtration, where such solution is feasible and when the system is properly installed, operated and maintained. Groups at high risk of these adverse health effects include the elderly (especially those living on their own), individuals with pre-existing illnesses, people living in overcrowded accommodation, and the socioeconomically deprived. A better understanding of how current and emerging building infrastructure design, construction, and materials may affect health in the context of climate change and mitigation and adaptation measures is needed in the UK and other high income countries. Long-term, energy efficient building design interventions, ensuring adequate ventilation, need to be promoted.

16As a major sector contributing to the UK's greenhouse gas (GHG) emissions, housing is an 17 important focus of Government policies to mitigate climate change. Current policy promotes 18 the application of a variety of energy efficiency measures to a diverse building stock, which 19 will likely lead to a wide range of unintended consequences. We have undertaken a scoping 20 review identifying more than 100 unintended consequences impacting building fabric, 21 population health and the environment, thus highlighting the urgent need for Government and 22 society to reconsider its approach. Many impacts are connected in complex relationships. Some 23 are negative, others possibly co-benefits for other objectives. While there are likely to be 24 unavoidable trade-offs between different domains affected and the emissions reduction policy, 25 a more integrated approach to decision making could ensure co-benefits are optimised, negative 26 impacts reduced and trade-offs are dealt with explicitly. Integrative methods can capture this 27 complexity and support a dynamic understanding of the effects of policies over time, bringing 28 together different kinds of knowledge in an improved decision-making process. We suggest 29 that participatory systems dynamics (PSD) with multi/inter-disciplinary stakeholders is likely 30 to offer a useful route forward, supporting cross-sectorial policy optimisation that places 31 reducing housing GHG emissions alongside other housing policy goals. 32 33 Introduction 34

Simulations using CONTAM (a validated multi-zone indoor air quality (IAQ) model) were employed to predict indoor exposure to PM2.5 in London dwellings in both the present day housing stock and the same stock following energy efficient refurbishments to meet greenhouse gas emissions reduction targets for 2050. To achieve these targets, measures were specified that reduced building permeability to 3m 3 m-2 hr-1 at 50 Pa, combined with the introduction of mechanical ventilation and heat recovery (MVHR) systems. It was assumed that the mean current outdoor PM2.5 concentration of 13μg.m-3 , decreased to 9μg.m-3 by 2050 due to emission control policies. Proper installation of MVHR systems with permeability reduction is associated with appreciable reductions in PM2. 5 exposure in both smoking and non-smoking dwellings. Modelling of the future scenario for nonsmoking dwellings predicts a reduction in annual average indoor exposure to PM2.5 of 24.0μg.m-3 (from 28.4 to 4.4μg.m 3) for a typical household member and a larger reduction of 52.8μg.m-3 (from 60.5 to 7.7μg.m3) for members exposed primarily to cooking-related particle emissions in the kitchen. Reductions in envelope permeability, without mechanical ventilation, produced a small increase (+5.4μg.m-3) in indoor PM2.5 concentrations. These estimates of changes in PM2.5 exposure were sensitive to assumptions about occupant behaviour, ventilation system usage and the distribution of input variables (+72% for non-smoking and +107% in smoking residences) but, if realised would result in significant health benefits.

Objective To investigate the effect of reducing home ventilation as part of household energy efficiency measures on deaths from radon related lung cancer.Design Modelling study. Setting England.Intervention Home energy efficiency interventions, motivated in part by targets for reducing greenhouse gases, which entail reduction in uncontrolled ventilation in keeping with good practice guidance.Main outcome measures Modelled current and future distributions of indoor radon levels for the English housing stock and associated changes in life years due to lung cancer mortality, estimated using life tables. ResultsIncreasing the air tightness of dwellings (without compensatory purpose-provided ventilation) increased mean indoor radon concentrations by an estimated 56.6%, from 21.2 becquerels per cubic metre (Bq/m 3 ) to 33.2 Bq/m 3 . After the lag in lung cancer onset, this would result in an additional annual burden of 4700 life years lost and (at peak) 278 deaths. The increases in radon levels for the millions of homes that would contribute most of the additional burden are below the threshold at which radon remediation measures are cost effective. Fitting extraction fans and trickle ventilators to restore ventilation will help offset the additional burden but only if the ventilation related energy efficiency gains are lost. Mechanical ventilation systems with heat recovery may lower radon levels and the risk of cancer while maintaining the advantage of energy efficiency for the most airtight dwellings but there is potential for a major adverse impact on health if such systems fail.Conclusion Unless specific remediation is used, reducing the ventilation of dwellings will improve energy efficiency only at the expense of population wide adverse impact on indoor exposure to radon and risk of lung cancer. The implications of this and other consequences of changes to ventilation need to be carefully evaluated to ensure that the desirable health and environmental benefits of home energy efficiency are not compromised by avoidable negative impacts on indoor air quality.

A number of studies have estimated population exposure to PM2.5 by examining modeled or measured outdoor PM2.5 levels. However, few have taken into account the mediating effects of building characteristics on the ingress of PM2.5 from outdoor sources and its impact on population exposure in the indoor domestic environment. This study describes how building simulation can be used to determine the indoor concentration of outdoor-sourced pollution for different housing typologies and how the results can be mapped using building stock models and Geographical Information Systems software to demonstrate the modifying effect of dwellings on occupant exposure to PM2.5 across London. Building archetypes broadly representative of those in the Greater London Authority were simulated for pollution infiltration using EnergyPlus. In addition, the influence of occupant behavior on indoor levels of PM2.5 from outdoor sources was examined using a temperature-dependent window-opening scenario. Results demonstrate a range of I/O ratios of PM2.5, with detached and semi-detached dwellings most vulnerable to high levels of infiltration. When the results are mapped, central London shows lower I/O ratios of PM2.5 compared with outer London, an apparent inversion of exposure most likely caused by the prevalence of flats rather than detached or semi-detached properties.