Extreme urban heat is no longer solely a physical hazard. For informal settlement communities in rapidly warming Southeast Asian cities, it also threatens mental well-being, social cohesion, and everyday survival. This thesis asks how extreme urban heat shapes the mental well-being of informal settlement communities in Quezon City, Philippines, and Bangkok, Thailand, and how residents, communities, and institutions respond. Using a mixed-methods design that combines spatial heat analysis, community-based evidence on lived experiences and coping strategies, and institutional and policy analysis, the study examines how extreme heat is distributed, experienced, and governed across these communities.

The thesis advances four interconnected findings. First, spatial analysis shows severe heat exposure in both cities, although the urban form of this exposure differs: Quezon City exhibits multi-nodal heat zones, whereas Bangkok shows a more concentrated heat cluster. In both cases, dense built environments, intense economic activity, and insufficient green space reduce natural cooling. Second, informal settlement communities in heat hotspots are disproportionately affected because extreme temperatures interact with socioeconomic precarity, housing insecurity, and structural barriers that limit adaptation. Third, heat exposure reshapes mental well-being through everyday trade-offs involving cooling deficits, economic strain, disrupted rest, anxiety, and isolation. Although social assets and informal mutual support help residents cope, such reliance can become emotionally exhausting when formal support is limited. Fourth, institutional responses remain fragmented, as existing policies and services rarely address the mental well-being dimensions of heat. Residents are therefore left to absorb risks that require more targeted urban planning, public health, and social protection measures.

The thesis contributes conceptually by positioning mental well-being as a central dimension of climate vulnerability in informal settlements. Empirically, it offers comparative evidence from Quezon City and Bangkok on place-specific patterns of exposure, coping, and institutional neglect. Practically, it calls for integrated urban policies that treat heat adaptation, mental well-being, green space, essential services, and community resilience as inseparable priorities. Protecting informal settlement communities from heat therefore requires more than cooling the city; it requires transforming the unequal conditions that render heat mentally, socially, and materially unbearable.