Physical effort during labour and behavioural adaptations in response to heat stress among subsistence farmers in Burkina Faso: a gender-specific longitudinal observational study

Background

Climate change-related heat stress disproportionately affects people in sub-Saharan Africa, impairing physical performance and increasing the risk of heat-related illnesses. Despite subsistence farming's key role in food security, scarce studies have measured farmers' physical effort during labour and the impact of behavioural adaptations to heat stress in real-life settings. This long-term, empirical study in Burkina Faso, one of the world’s resource-poorest countries, aimed to measure the heat stress-labour effort relationships among subsistence farmers, accounting for gender-related differences in labour roles.

Methods

We conducted a 1-year observational study using the Nouna Health and Demographic Surveillance System in Burkina Faso. We included participants who were healthy, actively practising farmers aged 20–45 years who were neither overweight nor underweight, had no chronic illnesses, no planned pregnancy or breastfeeding status, and resided within a 10-km radius of the weather station. Using research-grade wearables, we collected time-series data on environmental conditions, tracking indoor and outdoor wet-bulb globe temperature (WBGT) and physiological responses such as daily activity, estimated core body temperature, heart rate, and GPS-tracked movements. To operationalise physical effort during labour, we evaluated the physiological strain index (PSI; 0–10 scale with 10 being highest strain) and activity intensity during fieldwork, measured as functions of WBGT. Behavioural adaptations were measured using working patterns including work duration, break times, and seasonal work distribution. The main outcomes are compared across genders and complemented by analyses of monthly vital sign assessments and heat impact surveys.

Findings

Between Aug 9, 2021, and Aug 30, 2022, we recruited 39 subsistence farmer households (39 women, 39 men). One household dropped out of follow-up, and 38 households (76 participants [38 men, 38 women]) were included in the analysis. During fieldwork, PSI decreased as WBGT increased (–0·04 [95% CI –0·07 to –0·01], p=0·0056), suggesting adaptive pacing. Men reduced their average activity intensity once WBGT exceeded 27°C. By contrast, women experienced a higher PSI and activity intensity during fieldwork than men (PSI: 0·47 [0·07–0·87], p=0·025). Each 1°C WBGT rise extended daily working time by 12·3 min (95% CI 2·75–21·89, p=0·013) and participants shifted work patterns in response to heat stress by increasing break duration and shifting work to cooler times.

Interpretation

Heat stress significantly reduces the physical effort during labour. Self-employed subsistence farmers appear to adapt to heat stress by lowering labour intensity, redistributing tasks throughout the day, and shifting work to cooler months. As climate change worsens, these adaptations could become inadequate. Women, who often balance household chores and fieldwork, have limited pacing strategies, heightening their vulnerability to increasing heat stress. Our findings highlight the urgent need for targeted adaptation strategies to sustain physical effort during labour and protect vulnerable groups, especially women, from escalating heat stress.