Predicting indoor heat exposure risk during extreme heat events

Highlights

• We measure heat and humidity in 285 New York City residences in the summertime.

• Indoor conditions show between-home variability but respond to outdoor conditions.

• Heat wave simulations show that the indoor heat index can reach dangerous levels.

• Indoor heat danger is underappreciated and likely to increase with climate change.

Abstract

Increased heat-related morbidity and mortality are expected direct consequences of global warming. In the developed world, most fatal heat exposures occur in the indoor home environment, yet little is known of the correspondence between outdoor and indoor heat. Here we show how summertime indoor heat and humidity measurements from 285 low- and middle-income New York City homes vary as a function of concurrent local outdoor conditions. Indoor temperatures and heat index levels were both found to have strong positive linear associations with their outdoor counterparts; however, among the sampled homes a broad range of indoor conditions manifested for the same outdoor conditions. Using these models, we simulated indoor conditions for two extreme events: the 10-day 2006 NYC heat wave and a 9-day event analogous to the more extreme 2003 Paris heat wave. These simulations indicate that many homes in New York City would experience dangerously high indoor heat index levels during extreme heat events. These findings also suggest that increasing numbers of NYC low- and middle-income households will be exposed to heat index conditions above important thresholds should the severity of heat waves increase with global climate change. The study highlights the urgent need for improved indoor temperature and humidity management.

Introduction

Heat waves are typically defined as prolonged periods of elevated temperature and humidity (D'Ippoliti et al., 2010, Smith et al., 2013). These events are associated with increases in morbidity and mortality, and extreme heat waves can cause public health emergencies. In summer 2003, a 9-day heat wave in Western Europe caused between 50,000 and 70,000 excess deaths (Larsen, 2006, Robine et al., 2008). Although most analyses of historical and future heat-related mortality have focused on increases in outdoor ambient temperature, the majority of fatal heat exposures in the developed world occur indoors. In New York City (NYC), over 80% of heat strokes citywide have been attributed to exposure at home (New York City Department of Health and Mental Hygiene, 2013), and during the 2003 European heat wave, 50% of the observed fatalities in France occurred in homes (a figure that does not include deaths in hospitals that may have resulted from residential heat exposure) (Fouillet et al., 2006). For the United States (US) and Europe, climate models predict increases in the frequency and duration of extreme summertime temperatures (Duffy and Tebaldi, 2012, Karl et al., 2008), heat wave intensity and frequency (IPCC, 2007, Meehl and Tebaldi, 2004), and heat-associated morbidity and mortality (Hayhoe et al., 2010, Huang et al., 2011, Knowlton et al., 2007, Lin et al., 2012). An “analog city” approach has estimated, for example, that a heat wave analogous to the 2003 European event would lead to a tenfold increase in annual heat-related deaths in the city of Chicago (Hayhoe et al., 2010). Given this background, a thorough evaluation of key heat exposure environments is imperative; yet our understanding of heat and humidity conditions in the indoor residential environment is extremely limited.

The few studies that have attempted to characterize summertime conditions in the indoor residential environment (Arena et al., 2010, Franck et al., 2013, Mavrogianni et al., 2010, Mirzaei et al., 2012, Nguyen et al., 2013, Tamerius et al., 2013, White-Newsome et al., 2012, Wright et al., 2005) have demonstrated that temperature and humidity vary significantly across homes, despite similar outdoor conditions. Indoor environments are influenced by stable attributes (such as building type, window placement, and socioeconomic status) as well as behavioral factors such as cooking, bathing, and use of air conditioning (Tamerius et al., 2013, Yik et al., 2004). Like other health risks, heat stress is more likely to have adverse effects, including fatalities, among residents at the lower end of the socioeconomic spectrum (Harlan et al., 2006, Klinenberg, 2002). Improving public health measures designed to mitigate the effects of extreme heat necessitates accurate characterization of the range of heat and humidity conditions experienced in residential environments.

In this study, we analyze the association between indoor heat and humidity measurements recorded in 285 low- and middle-income New York City homes during the summer (June–September) and concurrent outdoor conditions. We use these observed relationships to build models to predict the response of indoor temperature and humidity to a range of outdoor conditions. Employing these models, we simulate expected indoor conditions during two extreme heat events: the 10-day 2006 NYC heat wave and a 9-day event that is an NYC analog of the 2003 Paris heat wave. In these simulations we employ a heat index (HI) measure that is commonly used to issue heat advisories in many US cities, including NYC (US Department of Commerce, 2010). The heat index is an important indicator of health risk because it combines temperature and humidity, both of which modulate the human body's ability to dissipate heat (Havenith, 2005).