Parenting in the Heat — Heatstroke Risk and Thermoregulation in Infants and Toddlers

Lead

On a 35°C (95°F) summer day, most parents feel a vague sense of danger when taking a young child outside — but it is hard to put a precise number on when that danger becomes real. Adults have internal alarms: sweat on the back, flushed cheeks, the body's own reporting. A preverbal infant has none of that, and cannot report anything. Compounding this, the infant's thermoregulatory system works differently from an adult's in ways that deserve more than the generic warning that "babies overheat easily."

The goal of this article is to translate "this seems risky" into "here is what is physiologically happening" — and to give that understanding a practical shape.

How infant thermoregulation differs from adults

Falk and Dotan, in a 2008 review published in Applied Physiology, Nutrition, and Metabolism, organized the relevant child-adult differences across several thermoregulatory dimensions [1].

Sweat rate. Children's sweat rate per unit body weight is lower than adults'. Because evaporative cooling depends on sweating, this limits heat dissipation in hot, humid conditions — precisely the conditions of a Japanese summer, where high humidity reduces sweat evaporation further.

Surface-area-to-body-weight ratio. Children have a larger body surface area relative to their mass. In dry heat, where radiation and convection carry heat away from the body, this ratio is an advantage. But when ambient temperature exceeds body temperature, the same large surface area becomes a liability: the environment transfers heat into the body at a higher rate.

Circulatory capacity. The cardiovascular response that moves heat from the body's core to the skin for dissipation is also more limited in children than in adults.

A 2011 American Academy of Pediatrics (AAP) policy statement on heat stress in exercising children — reaffirmed in 2025 — noted that under conditions of adequate hydration, children's thermoregulatory capacity is not as inferior to adults' as older textbooks suggested [2]. This is an important caveat, but it applies primarily to older children and adolescents who can exercise voluntarily and drink when thirsty. It does not apply straightforwardly to preverbal infants who cannot regulate their own fluid intake.

Why a parked car is a different kind of danger — the physics

"I'll only be a few minutes" is a judgment that the data do not support.

McLaren, Null, and Quinn (2005) measured cabin temperatures in a parked car across 16 separate days, with ambient temperatures ranging from 22°C to 35°C (72–95°F) [3]. The key findings:

• Regardless of outside temperature, interior cabin temperature rose by an average of 7°C (12.6°F) within the first ten minutes
• After one hour, the cabin was on average 22°C (40°F) hotter than outside, with a maximum observed difference of around 40°C
• 80% of the total temperature rise occurred within the first 30 minutes
• Car color and the presence of an (off) air conditioner made negligible difference
• Opening a window by a few centimeters did not meaningfully slow the temperature rise [3]

Even on a relatively mild day — 22°C outside — the interior of a locked car reached temperatures that created heat-illness risk. The study authors stated explicitly that a "cool day" affords no meaningful margin of safety [3].

For infants, the physiological consequences are more severe than for adults in the same environment: lower sweat capacity, higher surface-area exposure, and limited cardiovascular reserve combine to produce faster core temperature rise. Above 40°C body temperature, the risk of multi-organ damage and death rises sharply.

Indoor temperature and humidity guidelines

For indoor settings, the Japanese Ministry of the Environment's Heat Illness Prevention Manual recommends maintaining indoor temperatures below 28°C and relative humidity below 60–70% [5].

The relevant physical concept here is the Wet Bulb Globe Temperature (WBGT) — an index that integrates temperature, humidity, and radiant heat. Two rooms can have the same air temperature but very different WBGT values depending on humidity. High humidity inhibits evaporative cooling from sweat. In a room where an adult caregiver feels merely warm, an infant — less able to sweat and less able to report discomfort — may already be experiencing rising core temperature.

Concerns about electricity costs sometimes lead caregivers to delay turning on air conditioning. When an infant or toddler is in the room, the threshold for using cooling should be lowered accordingly.

Oral rehydration solution: what it does and when it is not enough

Oral rehydration solution (ORS) is a WHO-endorsed intervention for mild to moderate dehydration [4], and it is an important first-line option in the early home management of heat illness in infants and toddlers. For mild dehydration — the child is alert, drinking, and producing some urine — ORS is appropriate and effective.

ORS is not appropriate as a wait-and-see measure in the following situations. Seek medical care promptly when:

• Consciousness is altered (the child is limp, unresponsive, or difficult to rouse)
• Vomiting prevents the child from keeping fluids down
• No urine has been produced for an extended period
• Body temperature is 39°C or above and not coming down

Commercial sports drinks are not an adequate substitute for ORS: they contain too much sugar and too little sodium to function as a rehydration solution for young children. Use an infant-specific ORS product or one prescribed by a physician.

Priorities in order

When organizing summer precautions for an infant or toddler, the priorities run roughly as follows.

First: no child unattended in a parked car. The "I'll only be a minute" calculation fails the evidence test regardless of the day's temperature [3].

Second: manage the thermal environment from the infant's perspective. A caregiver who feels comfortable at a given temperature may be calibrating to an adult body. The infant's lower sweat rate and higher surface-area ratio mean their threshold is lower.

Third: provide fluids proactively. Infants cannot report thirst. Breast milk and formula meet hydration needs in ordinary indoor conditions, but during outdoor activity or in warm environments, additional water intake deserves active attention.

Fourth: know the signs. Flushed skin, dry skin, crying without tears, reduced wet diapers — when several of these appear together, move to a cool environment and consider contacting a pediatrician. The combination of signs matters more than any single indicator.

Understanding the physiology converts "this seems dangerous" into "here is what is going wrong right now" — a shift that makes the response faster and more targeted.

References

1. Falk B, Dotan R. Children's thermoregulation during exercise in the heat — a revisit. Appl Physiol Nutr Metab. 2008;33(2):420–427. PMID: 18347699. doi:10.1139/H07-185
2. Council on Sports Medicine and Fitness; Council on School Health. Climatic heat stress and exercising children and adolescents. Pediatrics. 2011;128(3):e741–e747. PMID: 21824876. doi:10.1542/peds.2011-1664 [Reaffirmed 2025]
3. McLaren C, Null J, Quinn J. Heat stress from enclosed vehicles: moderate ambient temperatures cause significant temperature rise in enclosed vehicles. Pediatrics. 2005;116(1):e109–e112. PMID: 15995010. doi:10.1542/peds.2004-2368
4. World Health Organization. The Treatment of Diarrhoea: A Manual for Physicians and Other Senior Health Workers. 4th rev. ed. Geneva: WHO; 2005. WHO/FCH/CAH/05.1. https://www.who.int/publications/i/item/9241593180
5. Ministry of the Environment, Japan. Heat Illness Prevention Environment and Health Manual 2022. https://www.wbgt.env.go.jp/pdf/manual/heatillness_manual_full.pdf [Japanese]
6. Fire and Disaster Management Agency, Japan. Heat illness emergency transport data, 2023. 2024. https://www.fdma.go.jp/disaster/heatstroke/items/heatstroke_geppou_2023.pdf [Japanese]