Menu
Elif Kılıç 1,*, Shruti M. Deorah 1, Dr. Ashok Gadgil 1,2
1 IECC, Goldman School of Public Policy, University of California Berkeley
2 Civil and Environmental Engineering, University of California, Berkeley
* Corresponding author – email
Protecting outdoor workers from dangerous heat requires metrics that capture how the human body actually experiences thermal stress during physical labor.
The heat indices currently used in India’s public health response, including temperature-based heatwave thresholds, wet-bulb temperature, and the NOAA Heat Index, were designed for general population guidance rather than occupational safety. These metrics treat all individuals as passive recipients of environmental heat, ignoring the substantial internal heat load generated by muscular work. A construction laborer mixing concrete or an agricultural worker harvesting crops may generate three to four times more metabolic heat than someone sitting indoors, yet existing indices provide identical risk assessments for both. This fundamental mismatch means that current heat warnings often arrive too late for workers, declaring “caution” when physiological danger is already present.
The Extended Heat Index for labor at metabolic rate N, designated EHI-N*, represents a new approach built from the physics of heat exchange in a human body with varying levels of muscular effort.
The model simulates the body’s thermal regulation system: blood carrying heat from working muscles to the skin surface, sweat evaporating to carry heat away, and the balance between heat production and heat loss that determines whether core temperature remains stable or begins to climb. What distinguishes EHI-N* from earlier physiological models is its incorporation of physical effort, solar radiation, and real-world biological limits. It explicitly accounts for metabolic heat production from labor (ranging from sedentary to heavy exertion) and radiant heat gain from direct sun exposure, both of which significantly increase cooling demands beyond what ambient temperature and humidity alone would suggest.
The following animation shows historical zones for the pre-monsoon season across India.
The naming convention communicates the specific scenario being modeled. The number following “EHI” indicates the metabolic intensity in METs, the standardized unit used in exercise physiology and occupational health. EHI-3 models light activity such as office work or slow walking. EHI-6 models the sustained heavy exertion typical of manual labor: digging, lifting, carrying loads, or operating heavy machinery. The asterisk appearing after some indices signals that direct solar radiation has been included in the heat load calculation, relevant for workers in unshaded outdoor environments. Indices without the asterisk assume full shade. All calculations use body dimensions representative of Indian adults (1.65 m height, 65 kg weight), though the underlying physics applies broadly. This specificity allows SHRAM to provide genuinely tailored guidance: the conditions that pose risk to a brick kiln worker differ meaningfully from those affecting a security guard, and EHI-N* captures these differences quantitatively.
Integrating EHI-N* into early warning systems could provide actionable thresholds to protect the large vulnerable group of outdoor workers, who continue with livelihoods involving heavy labor even when environmental conditions put their health at risk. This has relevance for various policy frameworks including occupational safety regulations, labor laws, employment guarantee schemes, etc.
Rather than presenting continuous numerical values that require expert interpretation, EHI-N* presents thermal stress as six discrete zones corresponding to distinct physiological states. This design reflects how the body’s cooling systems engage sequentially as heat load increases.
Zone 1 indicates cold stress requiring active heat conservation, a condition irrelevant to Indian summers.
Zone 2 represents the thermoneutral range where routine blood circulation to the skin maintains comfortable core temperature without active cooling.
Zone 3 marks the onset of sweating as the first active cooling response, but perspiration remains light and easily managed with normal fluid intake. These three zones represent conditions where outdoor work can proceed without special precautions beyond basic hydration.
Zone 4
The transition to Zone 4 signals a qualitative shift in physiological burden. Blood vessels dilate substantially to increase heat transport to the skin, the heart works harder to maintain this increased circulation, and sweat production rises to levels requiring deliberate attention to fluid and electrolyte replacement. These are not emergency symptoms but rather signals that the body is working hard to maintain thermal balance. Scheduled rest in shaded areas, frequent and periodic access to water, and monitoring by supervisors become important protective measures.
Zone 5
Zone 5 represents a more urgent state where the body approaches the ceiling of its cooling capacity. Sweat glands operate near or at their maximum output and skin blood flow reaches levels that compete with muscles for cardiac output. Symptoms intensify: heavy sweating, marked fatigue, muscle cramping, headache, dizziness, and racing pulse. Continuing heavy work in Zone 5 conditions courts serious heat illness, and the appropriate response is to reduce exertion, extend rest periods, and actively seek cooler environments.
Zone 6
Zone 6 describes conditions where the body’s defenses have been overwhelmed. Core temperature begins to rise because heat production exceeds maximum possible heat loss, and this initiates a cascade of dangerous health effects: cellular damage, enzyme dysfunction, and eventually organ failure. The symptoms reflect this systemic crisis: skin that feels hot and may be either bone-dry (when sweating fails) or drenched with ineffective sweat, mental confusion and disorientation, nausea and vomiting, breathing that becomes rapid and shallow, loss of coordination, and collapse. Without interventions such as rapid cooling and medical treatment, heat stroke can cause permanent disability or death within hours.
SHRAM (System for Heat Risk Assessment for Manual labor) translates EHI-N* into an actionable monitoring tool for protecting outdoor workers across India. The dashboard displays real-time heat stress conditions using data from India Meteorological Department weather stations, alongside 1- and 3-day forecasts for over 700 districts. Users can select different work intensities (EHI-3 for light work through EHI-6 for heavy agricultural labor) and toggle between shade and direct sunlight conditions to see how heat risk varies.
By providing district-level heat risk assessments in an accessible format, SHRAM enables employers, health officials, and workers themselves to make informed decisions about safe working hours during India’s increasingly dangerous summer heat waves. The dashboard serves both as an early warning system for identifying high-risk conditions and as a planning tool for scheduling outdoor work around the coolest parts of the day.
