However, forest loss can also impact human health by interacting with other forms of natural capital, such as air and water. Air and water quality are critical determinants of human health. According to the World Health Organization (WHO), 94 percent of the world's population suffer direct exposure to unsafe concentrations of particulate matter (PM), a standard indicator of air pollution. Air pollution can harm health in both the shortand long-term ( <mark>Juginović et al. 2021;</mark> Giani et al. 2020; Manisalidis et al., 2020; World Bank, 2016), while air purification can be extremely costly (Cohen et al., 2017; Domingo et al., 2021; Huangfu & Atkinson, 2020; Lelieveld et al., 2015; Ritchie & Roser, 2013). Polluted water can cause acute and chronic diseases, including diarrheal disease (DD) and stunting (Zaveri et al., 2020). Today, DD is among the leading causes of death among children under five (Liu et al., 2012; Prüss-Ustün et al., 2014). Moreover, an estimated 13.5 percent of global stunting prevalence is attributable to DD (Danaei et al., 2016), which is widely recognized as a major impediment to development (Galasso & Wagstaff, 2018).

Deforestation and its links to air pollution are often due to forest fires. For instance, prenatal exposure to the 1997 Indonesian forest fires and the resulting exposure to dangerous levels of air pollution have been found to adversely impact child height-for-age scores as well as later life wages for the approximately 1 million Indonesian workers born during this period (Tan-Soo & Pattanayak, 2019). Air pollution generated from Indonesian forest fires can also significantly impact neighboring countries through severe episodes of “seasonal haze”, resulting in increased air-related diseases, including acute respiratory infections (ARI) in Singapore (Sheldon & Sankaran, 2017). Forest fires can also have global impacts. For example, reductions in deforestation rates and associated deforestation fires in the Amazon rainforests, known as “the lungs of the planet”, can significantly reduce PM, preventing thousands of excess deaths (Cobelo et al., 2023; Reddington et al., 2015).

However, fewer studies have investigated the causal links between the loss of forest cover and the hydrological services they provide and human health. Empirical studies in forest science and hydrology show that deforestation can impact access to clean water through changes in water yield and quality. Although forests— especially fast-growing young plantations—may use more water than older, mixed forests and therefore reduce freshwater availability, there is growing evidence that, with proper management, forests can help enhance the resilience of water supplies (Miller et al., 2020). Forest loss upstream can also decrease soil infiltration of water and the removal of pathogen, and increase soil erosion (Filoso et al., 2017; El Kateb et al., 2013; Zheng, 2006), which can result in lower stream water loads of Escherichia coli, higher pH of water and lower stream water turbidity (Cunha et al., 2016). Such water conditions can also render point-of-use chlorination less effective especially for households in low-income countries who cannot afford expensive water treatment (Amrose et al 2015).

Herrera et al. (2017) use a multi-country database to explore the connections between deforestation, watershed conditions, and childhood DD and find that higher upstream tree cover is associated with a lower probability of DD downstream. This effect is significant for rural households but not urban ones, suggesting differing dependence on watershed conditions. In rural areas, the effect of a 30 percent increase in upstream tree cover is similar in magnitude to the effect of improved sanitation but smaller than for improvement of water sources, wealth, or education. Country-level studies further provide evidence that deforestation reduces access to clean drinking water (Mapulanga and Naito, 2019) and undermines the effectiveness of point-of-use chlorination (Rasolofoson et al., 2021), thus further increasing the risk of DD.

Given the importance of upstream forest cover loss to DD, it is possible that upstream forest cover loss could also influence stunting rates through the water channel. Past work has shown that repeated bouts of DD increase the probability of nutritional deficiencies in children, thus stunting child development (Zaveri et al., 2020). Exposure to water pollution can also adversely affect birth outcomes by lowering infant birthweight (Jones, 3