Ambient air temperatures in Mexico have broken record highs in 2024. Solutions are needed to build heat resilience in communities and adapt to increasing heat from climate change. Sunlight-reflecting cool roof coatings may passively reduce indoor temperatures and energy use to protect home occupants from extreme heat. Occupants living in poor housing conditions in the northern zone of Mexico are susceptible to increased heat exposure.

Heat exposure can instigate and worsen numerous physical, mental and social health conditions. The worst adverse health effects are experienced in communities that are least able to adapt to heat exposure. By reducing indoor temperatures, cool roof use may promote heart health, sleep and physical activity in household occupants.

The long-term research goal of the investigators is to identify viable passive housing adaptation technologies with proven health benefits to reduce the burden of heat stress in communities affected by heat in Mexico. To meet this goal, the investigators will conduct a randomized controlled trial to establish the effects of cool roof use on heart rate, sleep and physical activity in Colima, Mexico.

Increasing heat exposure from climate change is causing and exacerbating heat-related illnesses in millions worldwide - particularly in low-resource settings. June 2024 was the 13th consecutive hottest month on record globally - shattering previous records. Heat exposure can instigate and worsen health conditions, including cardiovascular, metabolic, endocrine and respiratory disease, heat-related illnesses, pregnancy complications, and mental health conditions. Adaptation is essential for protecting people from increasing heat exposure. The built environment, especially homes, is ideal for deploying interventions to reduce heat exposure and accelerate adaptation efforts. However, there currently is a lack of evidence on a global scale - generated through empirical studies - guiding the uptake of interventions to reduce heat stress in low-resource settings.

Cities and towns of LMIC are among the most vulnerable to the adverse impacts of climate change and are likely to experience increases in ambient air temperature over the coming decades. People in Colima are exposed to heat and low humidity year-round. The combined burden of heat and NCDs places Colima's population at greater risk of adverse health effects from heat extremes.

Sunlight-reflecting cool roof coatings passively reduce indoor temperatures and lower energy use, offering protection to home occupants from extreme heat. The investigators therefore aim to conduct a randomized controlled trial investigating the effects of cool roofs on heart rate, sleep and physical activity using smartwatches in Colima.

The trial will quantify whether cool roofs are an effective passive home cooling intervention with beneficial health effects for vulnerable populations in Colima. Findings will inform regional and global policy responses on scaling cool roof implementation to protect people from increasing heat exposure driven by climate change.

Inclusion Criteria:

• Permanent household resident

Exclusion Criteria:

• Roof damage, inaccessible or instability of roof adversely affecting cool roof coating application.
• Participant unable to provide written/verbal informed consent. Participants will be excluded if they are not willing or able to wear a smartwatch.
• Participants will be excluded if they do not have a smartphone with an internet connection that can connect to the smartwatch.
• Only one participant per household.

Data that can be shared unconditionally underpinning the published research articles will be made available to other researchers at the time of publication, and data will be linked via the article DOI. Data that cannot be unconditionally shared upon publication owing to confidentiality or data protection requirements will be identified as such and a contact email will be provided in relevant publications for data access enquiries by other researchers. Individual names of study participants and identifying factors will be removed prior to data sharing.

It is expected that demographic data of people at the study sites (family size and composition, basic socioeconomic indicators) may contain personally identifiable information and location data. All such data will be removed prior to storage on online data repositories and therefore will be available to be publicly shared at the time of publication of manuscripts.

At the time of publication.

Trial participants will be aware of the intervention to which they have been allocated, and the research fieldworkers will be aware of the intervention allocation. The trial steering committee members and trial statistician will remain blinded until the end of trial period and data collection.