Heat and Cold Stress: CIH Reference

01 Start With the Assessment Question

The question every thermal assessment starts from, and the chain that follows it.

Can the worker release enough heat—or retain enough heat—to keep body temperature within a tolerable range during the actual task?

A core temperature of 37°C / 98.6°F is a useful reference, not an exact normal value for every person at every moment. Distinguish environmental heat stress from the resulting heat strain. ACGIH states the goal of its heat TLV as keeping core temperature within about ±1°C of 37°C (roughly 36–38°C), and says this range can be exceeded in selected populations with environmental and physiological monitoring. It is a TLV goal, not a legal requirement to hold every worker at an exact temperature. CCOHS: heat health effects

02 Heat Balance: Follow the Energy

Storage equals metabolic heat plus gains minus losses, with consistent signs and units.

Use a consistent sign convention. Here, exchange terms are positive when they represent heat leaving the body:

All terms must use the same rate units, such as watts. Respiratory exchanges can be included in the appropriate terms or shown separately in a more detailed model; do not count them twice.

For constant rates over a time interval:

Watts are joules per second. Thus, $\dot S$ is a rate, while $\Delta S$ is stored energy. Writing $\Delta S$ beside terms in kcal/hour without defining the time basis mixes energy and power.

Some references instead write $S=(M-W)\pm C\pm R\pm K-E$. This expresses the same physical accounting when gains and losses are assigned consistently. NIOSH heat criteria, Chapter 3

Equation card: Heat balance, with a worked explanation in the CIH equation reference.

Worked example: a worker near a furnace

Assume the following whole-body rates:

• Metabolism: 400 W; external mechanical work: 50 W.
• Convection removes 40 W; radiation adds 80 W.
• Conduction is negligible; evaporation removes 250 W.

Over 30 minutes:

Interpretation: heat is accumulating. The calculation does not, by itself, predict a particular core temperature or time to heat stroke. Real exchange rates and physiological responses change over time.

Memory aid: metabolic heat + environmental gains − heat losses = storage.

03 Why Each Heat-Transfer Mechanism Matters

Convection, radiation, conduction and evaporation, and three corrections to common shortcuts.

These mechanisms operate together. Airflow can change convection and evaporation at the same time. Skin blood flow brings heat toward the surface; whether it leaves the body depends on the surrounding conditions and clothing. CCOHS: heat exchange

Three corrections to remember

1. Relative humidity is not the entire evaporation calculation.

The relevant driving difference is approximately:

Ambient RH describes saturation at the air temperature. Therefore, saturated air cooler than wet skin can still have a lower vapor pressure than the skin surface. The conclusion that “100% RH always means zero skin evaporation” does not follow. This is a physical inference from the vapor-pressure relationship. Clothing resistance and skin wetness also matter. NIOSH heat criteria, Section 3.2.3

2. A fan does not remove the radiant source. It may still help through other exchange mechanisms. Shielding, source insulation, separation, remote operation, and recovery in a cool area address different parts of the overall heat load.

3. Do not use 95°F as a universal fan on/off threshold. It is an assumed skin temperature in some simplified models. Above skin temperature, faster air can increase convective heat gain, while also affecting evaporation. Evaluate the net effect, humidity, clothing, and actual conditions; extremely hot air is not a reliable cooling supply. CCOHS: heat controls

04 Formula and Unit Audit: Expressions That Mislead

A correct-looking equation with mismatched units can give a seriously wrong answer.

Some study notes give $C$, $R$ and $E$ expressions with the units shown below. They should not be used with those units.

These are simplified relationships for a reference worker and specified clothing, not universal heat-transfer laws. The fixed skin temperature and skin vapor pressure are assumptions. Preserve the complete equation, units, sign convention, and applicability when solving a problem. NIOSH heat criteria, printed pages 14–15

Useful constants and conversions

The clo conversion is documented in NIOSH heat criteria. Conversion examples use ordinary unit arithmetic.

05 Measurement: Choose the Instrument for the Question

Each tool answers a different question, and none of them alone gives heat strain.

Place the instrument where the worker actually experiences heat, including sun or process radiation. Follow setup, calibration, stabilization, and maintenance instructions. Weather-station data may not represent a furnace platform, enclosed cab, or sheltered work area. OSHA: heat hazard recognition

Sensor memory aid (original): Wet wick → black globe → dry air. In sun: 7–2–1. Without sun: 7–3. The natural wet-bulb sensor is a wet wick exposed to local natural conditions, not wet clothing and not automatically a psychrometric wet bulb. The globe responds to radiation and convection, so it matters indoors near hot processes as well as outdoors. The dry bulb measures air temperature and appears as a separate term only in the direct-solar-load equation. Do not conclude that air temperature is physically irrelevant indoors: the two equations are index formulations, not percentages of heat flowing through the body.

Quality checks: record time, location, task, clothing, instrument identification, stabilization, and unusual conditions. For wick instruments, check water and wick condition. For instruments estimating wet-bulb values electronically, confirm suitability for the measurement method.

06 WBGT Calculations and Interpretation

Two equations, three worked examples, and the step from a number to a decision.

Use $T_{nwb}$ for natural wet-bulb, $T_g$ for globe, and $T_{db}$ for dry-bulb temperature.

Without direct solar load:

With direct solar load:

An indoor furnace can still produce strong radiant exposure; “without solar load” does not mean “without radiation.” The globe component remains in both equations. Use consistent units and the units required by the assessment criteria. CCOHS: WBGT calculations

Equation cards: WBGT with no solar load (indoors) and WBGT with solar load (outdoors), each with notation and an explanation.

Example A: indoor process heat

Natural wet bulb = 26°C; globe = 38°C:

Example B: work in direct sunlight

Natural wet bulb = 25°C; globe = 40°C; dry bulb = 32°C:

Example C: nighttime maintenance crew

Natural wet bulb = 25°C; globe = 35°C; dry bulb = 30°C. No direct sunlight reaches the work area. Which equation applies, and what is the WBGT?

The solar-load equation would give $0.7(25)+0.2(35)+0.1(30)=27.5°C$, so choosing the wrong equation changes the answer. The elevated globe reading at night can reflect nearby process heat. Daytime air temperature plays no part in this measurement set.

Example D: time weighting

For an illustrative cycle with 45 minutes at WBGT 30°C and 15 minutes at WBGT 24°C:

If the corresponding metabolic rates are assumed to be 415 W and 115 W:

These calculations demonstrate weighted averages. They do not establish that the proposed cycle is safe. Use the averaging period, clothing treatment, and exposure pattern specified by the selected assessment method; do not average away a dangerous short exposure or symptoms.

From measurement to decision

1. Identify the task, location, and exposure duration.
2. Measure representative WBGT, including changing conditions.
3. Estimate metabolic workload from actual activity and pace.
4. Apply the appropriate clothing adjustment where the method permits it.
5. Identify acclimatization status.
6. Compare using the applicable exposure guidance and its assumptions.
7. Select controls, monitor workers, and reassess.

ACGIH distinguishes the Action Limit and TLV for different acclimatization assumptions. NIOSH uses Recommended Alert Limit (RAL) and Recommended Exposure Limit (REL) terminology. Consult the actual selected framework; the names and limits are not interchangeable regulatory labels. OSHA Technical Manual: heat stress

The ACGIH heat assessment workflow

ACGIH's own decision chart (Figure 1 of its Heat Stress and Heat Strain TLV) is the workflow to know. Start it when a qualitative assessment suggests possible heat stress, workers report discomfort, or professional judgment points to heat stress. Not every case passes through every step.

Note the logic: above the Action Limit but below the TLV means general controls; above the TLV for acclimatized workers means physiological monitoring is the only way to show adequate protection. This is the chart ACGIH publishes, not a four-tier system. Sources: ACGIH Heat Stress and Heat Strain TLV (booklet pages, edition not printed on the pages read) and CCOHS: heat assessment. ACGIH TLV pages

Screening criteria: ACGIH Table 2 (WBGT, °C)

Values are WBGT with the clothing adjustment already added, for an 8-hour day in a 5-day week with conventional breaks. A dash means ACGIH gives no value there; do not fill it in.

• TLV applies to heat-acclimatized workers; the Action Limit applies to unacclimatized workers.
• Values are given to the nearest 0.5°C. Where work and rest happen in different places, use time-weighted averages.
• The blanks exist because heavy and very heavy work among less fit workers carries strain risk regardless of WBGT: use detailed analysis or physiological monitoring instead.
• The table is a screening tool, more protective than the TLV or Action Limit curves, and is not a prescribed work/rest schedule.
• Acclimatized, for the TLV: a recent history of at least two continuous hours of heat-stress exposure, for example on 5 of the last 7 to 10 of the last 14 days. Loss begins when exposure stops, is noticeable after about four days, and can be complete in three to four weeks.

Metabolic rate categories used with the table (Table 3): rest 115 W, light 180 W, moderate 300 W, heavy 415 W, very heavy 520 W. Sources: ACGIH TLV pages above; CCOHS table, adapted from ACGIH 2026.

Limiting heat strain: ACGIH Table 4

Any one of these may mark excessive heat strain, and exposure is discontinued so the worker can recover:

A worker may be at greater risk (these are risk indicators, listed separately from the stop criteria) if profuse sweating is sustained over hours, if weight loss over a shift exceeds 1.5% of body weight, or if 24-hour urinary sodium excretion is below 50 mmol.

Emergency response (ACGIH wording in brief): a worker who is disoriented or confused, or has inexplicable irritability, malaise or chills, is removed to rest in a cool place with rapidly circulating air and kept under skilled observation. Absent medical advice to the contrary, treat it as an emergency with immediate transport to a hospital. Never ignore signs or symptoms of heat-related disorders. ACGIH TLV pages

07 Clothing: clo and CAF Answer Different Questions

Insulation is not the same as an adjustment to a WBGT assessment.

CAF is not a new name for clo. A zero CAF means the ensemble is the assessment baseline; it does not mean the clothing provides zero insulation.

The following public OSHA table is adapted from NIOSH 2016. It is a study reference, not a claim to reproduce the latest ACGIH table. OSHA clothing adjustments

Do not add arbitrary garment factors together. Match the complete ensemble to the assessment method. Do not apply the limited-use-coverall factor indiscriminately to a fully encapsulating Level A ensemble; specialized assessment and monitoring are needed.

Worked example: for Example B and the specified double-layer ensemble:

The air has not physically warmed by 3°C. The adjustment changes how the measured environment is evaluated for that clothing. A 3°C difference equals 5.4°F, not 37.4°F.

08 Physiology, Acclimatization and Hydration

Why the body responds as it does, and the NIOSH example progressions.

Why the body responds this way

Heat adaptation includes expansion of plasma volume, not “increased plasma output.” Plasma is a blood compartment, not an output stream. NIOSH heat criteria, Section 4.1.5

Acclimatization plan

NIOSH describes adaptation over approximately 7–14 days. Benefits include earlier and more effective sweating, reduced electrolyte loss, and lower heart rate and core temperature for comparable work.

These are exposure progressions, not proof of full physiological adaptation by day 4 or 5. Absence for a week or more can reduce adaptation. Air-conditioned recovery breaks do not erase acclimatization. NIOSH: acclimatization

Hydration

For moderate activity in heat lasting less than two hours, NIOSH recommends approximately one 8-US-fluid-ounce cup (about 240 mL) every 15–20 minutes. Several hours of sweating may call for balanced electrolyte replacement. NIOSH advises that intake generally should not exceed six cups per hour; excessive fluid intake is not a prevention strategy. Individual medical restrictions require individualized planning. NIOSH: workplace recommendations

Study calculation: 240 mL every 20 minutes is about 720 mL/hour; every 15 minutes is about 960 mL/hour. These are conversions of the example guidance, not mandatory amounts for every worker.

Consider illness, medications, pregnancy, cardiovascular or kidney conditions, fitness, hydration, and acclimatization when planning work. Body shape alone is not a reliable fitness-for-heat decision. CCOHS: individual susceptibility

09 Recognize Heat Illness and Act

Neurological change in the heat means emergency help and immediate cooling.

Confusion, slurred speech, seizures, or collapse during heat exposure require an emergency response. Do not wait for dry skin, loss of sweating, or a temperature of 106°F.

Illness descriptions: NIOSH: heat-related illnesses.

For suspected heat stroke: remove excess outer clothing when feasible, cool immediately with cold-water immersion when it can be done safely, or use other active cooling such as wetting the body and circulating air while emergency help is coming. Maintain supervision and airway safety. Do not give oral fluids to someone confused, unconscious, seizing, vomiting, or unable to swallow safely; drinking must not delay emergency cooling. OSHA: heat first aid, CCOHS: heat first aid

Memory aid: neurological change in the heat → emergency help + immediate cooling.

10 Select Controls by the Mechanism

Match each problem to the part of the heat balance the control changes.

Control framework: OSHA Technical Manual, NIOSH workplace recommendations.

Do not remove required chemical, biological, electrical, or impact protection merely to reduce heat. Solve the combined hazards through equipment selection, engineering, work organization, and appropriate recovery.

Match the control to the cause

Hot workpiece or furnace   → radiant exposure            → shield or insulate the source
Confirmed steam leak       → added moisture and heat     → repair the leak
Heavy handling             → metabolic heat              → mechanize or reduce effort
Sweat cannot evaporate     → clothing or humid-air limit → assess compatible controls

Establish the actual source before choosing the control. High humidity alone does not prove a steam leak. A fan does not always reduce net heat load, and a cooling vest is personal cooling equipment, not an engineering control. These are examples to evaluate, not a fixed package. CCOHS: heat controls

Structure of a heat stress management program

ACGIH recommends starting a program when heat stress exceeds the Action Limit, or when clothing limits heat loss. It offers the elements below (its Table 5) as considerations for the industrial hygienist's judgment; they are not part of the TLV itself and not a legal checklist. Monitor heat stress (Table 2) and heat strain (Table 4) to confirm the controls work.

Good practice that a program also covers (not on the ACGIH list): roles and responsibilities, a gradual acclimatization schedule (NIOSH example in section 8), an emergency plan that includes cooling before transport, and records of measurements, training and incidents. Training or review frequencies are legal requirements only where the jurisdiction says so. Check the local regulation. ACGIH TLV pages, NIOSH workplace recommendations

Heat and chemicals. Warm skin can influence dermal absorption, but the effect depends on the substance, the skin condition and the exposure. Keep skin absorption separate from permeation through a glove. Do not turn any single "doubles per 10°C" rule into a prediction for real workers; if a problem stipulates one, treat it as an assumed model. NIOSH: skin exposures

11 Other Indices: What Question Do They Answer?

WBGT, heat index, HSI and older indices are not interchangeable.

For the traditional HSI concept:

If required evaporative cooling is 300 W and estimated maximum capacity is 250 W, HSI = 120%. This means the modeled evaporation requirement exceeds the modeled capacity. It does not mean 120% body temperature, nor does HSI below 100% prove that every worker is safe. NIOSH heat criteria, Section 9.2.2

Heat index omits important workplace factors such as process radiation, workload, and clothing. OSHA: heat hazard recognition

12 Cold Stress: The Same Balance, Opposite Problem

Losses can exceed heat production; wetness, wind and contact drive the risk.

In cold conditions, losses can exceed metabolic heat production. Vasoconstriction conserves heat in the core but reduces warmth at the extremities; shivering increases metabolic heat production.

The statement “water conducts heat about 23 times faster than air” is not a whole-body cooling multiplier. Actual cooling depends on temperature, motion, contact, insulation, and geometry. Do not calculate immersion survival time from that ratio. CCOHS: cold health effects

Cold illnesses and injuries

Cold injury distinctions: NIOSH: cold-related illnesses.

Corrections: shivering can begin before core temperature reaches 35°C. Do not memorize 33°C as a universal definition of “severe hypothermia.” Act on suspected illness without waiting to assign a temperature stage. Hypothermia can occur above freezing, especially when someone is wet. Handle the person gently; warm beverages are appropriate only when alert and able to swallow safely. CDC: hypothermia

Whole-body cooling and local tissue injury are different problems: hypothermia is core heat loss and can occur above freezing, frostbite is local freezing, and trench foot and chilblains arise without freezing. Do not stage a cold victim from symptoms or a temperature band alone. Rarer conditions such as frostnip and cold urticaria are not covered here because this review did not confirm a source for their classification.

For frostbite, avoid unnecessary walking on affected feet, rubbing, massage, and direct heat from a stove or heating pad. Numb skin can burn without warning. Obtain medical care and check for coexisting hypothermia. CDC: frostbite

13 Wind Chill and Cold Controls

A cooling equivalent, not an air temperature, with the NWS equation and controls.

Wind chill describes the cooling effect of wind on exposed skin. It is not the measured air temperature, body temperature, or a complete assessment of clothing, wetness, and workload.

The U.S. National Weather Service equation is:

Here, $T$ is air temperature in °F, and $V$ is wind speed in mph. Its stated range is air temperature at or below 50°F, with wind above 3 mph. Do not insert °C or km/hour into this version. NWS: wind chill questions

Example: at 10°F and 15 mph, calculated wind chill is approximately −7°F. The air is still 10°F. Wind alone makes an object approach air temperature faster; it does not make that object reach the wind-chill temperature.

Cold-work guidance with its limits. CCOHS says that where fine manual dexterity is not required, gloves should be used below 4°C for light work and below −7°C for moderate work. It cites ACGIH (2024) for a work-warming regimen when work is continuous at wind chill of −7°C or below, and for added measures (such as constant observation by a supervisor or buddy) at −12°C or below. CCOHS's separate work/warm-up schedule applies to any four-hour period of moderate or heavy activity with normal breaks every two hours; it is not a statement about every shift. These are guidance values with stated conditions, not universal legal triggers. Check the requirement in the workplace's own jurisdiction. CCOHS: cold control measures, CCOHS: cold overview

Cold stress numbers in one place

These come from the IHSA construction manual (2024 edition, adapted from ACGIH) and CCOHS (citing ACGIH 2024). They are written as "should" guidance. The IHSA states that Ontario has no legislated exposure limits for working in cold, so a number here is not automatically a legal trigger.

The ACGIH work/warm-up schedule (IHSA Table 7-1) applies to moderate or heavy work in any four-hour period, with warm-up breaks of 10 minutes in a warm place and a normal break after two hours; it assumes dry clothing and is stepped one level more protective when physical activity is limited. It starts at air temperatures of about −26 to −28°C on a sunny day with no noticeable wind and shortens the allowed work period as wind rises, ending in "non-emergency work should stop" at the coldest, windiest combinations. Read the table itself before using it. IHSA cold stress chapter

Choose clothing for both the work period and the recovery period. Heavy activity can wet clothing with sweat, then stopping work reduces metabolic heat while the worker remains damp. NIOSH: preventing cold-related illness and injury

14 Final Memory Table and Practice Checks

One line per topic, then seven questions to apply it.

Master thermal numerical targets

Learn these as a set, with their qualifiers. "Exam use" below means they are the figures worth knowing; the qualifier is part of the answer.

Heat

Cold

Edition notes: the heat TLV values come from ACGIH's Heat Stress and Heat Strain pages (edition not printed on the pages read; the Table 2 values match the CCOHS table adapted from ACGIH 2026). Cold values come from IHSA (2024) and CCOHS. Confirm against the current TLV booklet before relying on any figure for an exam or a workplace decision.

Apply the knowledge

1. A worker is sweating heavily but becomes confused. Is heat stroke excluded?
   No. Activate emergency response and start cooling.
2. A hot furnace is indoors. Which WBGT equation applies?
   Without direct solar load: $0.7T_{nwb}+0.3T_g$. Indoor radiation still affects the globe.
3. Measured WBGT is 27°C and the applicable CAF is 3°C. What is effective WBGT?
   30°C. This is an assessment adjustment, not a measured air-temperature change.
4. A fan is added in air hotter than skin. Has heat stress necessarily decreased?
   No. Consider increased convective gain alongside any change in evaporation.
5. A worker rests in the same hot area. Is that equivalent to recovery in a cool room?
   No. Rest reduces metabolic heat, while the recovery environment determines external exchange.
6. A wet worker becomes confused at an air temperature above freezing. Can this be hypothermia?
   Yes. Wetness and prolonged cooling can create risk above freezing.
7. Why should the C, R and E formulas in Section 4 not be used with their original units?
   Their coefficients, airflow units, and heat-rate units were inconsistent. A correct-looking equation with mismatched units can produce a seriously wrong answer.
8. A night crew has natural wet bulb 25°C, globe 35°C and dry bulb 30°C, with no direct sun. What is the WBGT, and why not use the dry bulb?
   $0.7(25)+0.3(35)=28.0°C$. Without direct solar load the dry bulb is not in the equation; the globe still captures process radiation.
9. A work area is humid and hot. A colleague orders a steam-leak repair before anyone has looked. What is wrong?
   Humidity alone does not show a steam leak. Identify the actual source (radiant surface, moisture source, workload, clothing) first, then match the control to it.
10. A 50-year-old worker has a sustained heart rate of 135 bpm for several minutes and a normal core temperature. What does Table 4 say?
    The limit is 180 − 50 = 130 bpm, so 135 bpm sustained marks excessive strain (for a person with assessed normal cardiac performance): discontinue exposure and let the worker recover.
11. At one minute after a peak effort a worker's heart rate is 115 bpm. Stop criterion met?
    No. The ACGIH recovery criterion is above 120 bpm, not 110.
12. Screening Table 2 has no value for heavy work at 75–100% work. May we use the nearest cell?
    No. The blank is intentional: use detailed analysis or physiological monitoring for that case.

Verification scope

On October 9, 2026 the ACGIH heat stress and strain values (Tables 1 to 5 and Figure 1) were read from the booklet pages, the Table 2 values were compared with the CCOHS table adapted from ACGIH 2026, and the cold values were checked on the IHSA 2024 manual and CCOHS. The key figures (the NIOSH heat-exchange equations and clothing adjustment table, the NIOSH acclimatization and hydration guidance, and the NWS wind-chill equation) were checked against the primary documents on October 4, 2026; the other concepts follow the linked OSHA, NIOSH, CDC, CCOHS, and NWS materials. Original worked examples were calculated separately. Historical indices are retained for recognition; they are not presented as preferred modern field methods. Current proprietary ACGIH limits and jurisdiction-specific legal requirements were not reproduced or independently certified in this study reference.