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Introduction
Over 2.8 billion people are exposed to household air pollution from cooking and heating with solid fuels, which include biomass (e.g., wood, crop residues, animal dung, charcoal) and coal (Bonjour et al. 2013). Household air pollution comprises many pollutants (Zhang and Smith 2007; Naeher et al. 2007) and is a leading health risk factor, annually responsible for an estimated 2.9 million premature deaths (GBD 2013 Risk Factors Collaborators et al. 2015). Two widely studied air pollutants from solid fuel combustion are particulate matter (PM) and carbon monoxide (CO). Strong epidemiologic and experimental evidence point to the mass of PM with a diameter ≤2.5 μm (PM2.5) as a pollutant that is causally associated with many health outcomes (Pope and Dockery 2006; U.S. EPA 2009) and is likely a strong driver of many health effects associated with household air pollution (Brook et al. 2010; WHO 2014). Evidence for adverse health outcomes related to low-to-moderate CO exposure is sparse and less consistent, with associations between infant low birth weight and women’s CO exposure during pregnancy demonstrated in some studies (Ritz and Yu 1999; Ha et al. 2001; Gouveia et al. 2004; Salam et al. 2005), but not in others (Alderman et al. 1987; Koren et al. 1991; Chen et al. 2002; Parker et al. 2005; Wylie et al 2016). In epidemiologic and exposure studies of household air pollution, including those evaluating maternal exposure and birth outcomes (Thompson et al. 2011; Dix-Cooper et al. 2012), CO exposure is usually measured as a surrogate of PM2.5 exposure (Balakrishnan et al. 2011; Clark et al. 2013).
Accurate exposure assessment is the basis for evaluating exposure–response relationships (Armstrong 1998, 2004), and in the context of household air pollution, critical to interpreting the effectiveness of stove-fuel interventions (Peel et al. 2015). Direct measurement of personal exposure to PM2.5 mass is considered the “gold standard” in epidemiologic studies (Smith 1993; Northcross et al. 2015), but is challenging to measure in large populations (Northcross et al. 2015) and in infants (Naeher et al. 2001; Dionisio et al. 2008). Questionnaires and cooking area PM2.5 have been used alone or in combination as surrogates but were poorly associated with personal PM2.5 exposure in validation studies (Ezzati et al. 2000; Bruce et al. 2004; Cynthia et al....