Temperature and Humidity Conditions Favorable for Humans in Summer and Winter: Physiology, Thermoregulation, and Comfort

Introduction: Homeostasis in a Changing Environment

The human body maintains a constant internal temperature of about 36.6-37.0°C regardless of external conditions, meaning it is warm-blooded. This is achieved through a complex thermoregulation system, the key element of which is heat exchange with the environment. Favorable temperature and humidity parameters are not universal numbers but a dynamic range in which the thermoregulation system works efficiently without excessive strain on the cardiovascular and respiratory systems, providing a subjective feeling of comfort. These parameters differ in summer and winter due to different clothing, activity levels, and acclimatization of the body.

Physiological Basis of Comfort: Balance of Heat Production and Heat Loss

Heat loss occurs through four main pathways:

Convection (about 30%) — heat transfer to the air flowing over the skin.

Radiation (about 45%) — emission of infrared rays.

Evaporation (about 20%) — sweating.

Conduction (insignificant) — contact with colder objects.

Air humidity critically affects the efficiency of evaporative cooling. At high humidity, sweat does not evaporate but runs off the skin, failing to perform its cooling function, leading to overheating. At very low humidity, excessive evaporation of moisture from mucous membranes and skin occurs, causing dryness and discomfort.

Favorable Summer Regime: Emphasis on Evaporative Cooling

In warm seasons, when the body is set to dissipate excess heat, optimal parameters shift.

Air temperature: For rest in light clothing (shorts, T-shirt), the optimal range is 23-26°C. Within this interval, thermoregulation is mainly by convection and radiation without active sweating. During physical activity, the optimal temperature lowers (20-23°C) to compensate for increased heat production.

Relative air humidity: A key parameter. The optimal range is 40-60%. At these values, sweat evaporation occurs efficiently.

Above 70%: Even at 26-27°C, a feeling of stuffiness and overheating arises because sweat does not evaporate. The Heat Index used by meteorologists shows that at 85% humidity and 30°C, subjective perception is equivalent to 38°C of "dry" heat.

Below 30%: Air feels dry, dehydration accelerates, mucous membranes of the respiratory tract dry out, increasing the risk of respiratory infections.

Adaptation example: In traditional architecture of hot humid countries (e.g., Southeast Asia), houses are built on stilts, providing cross ventilation for maximum convection. In dry hot regions (Middle East), thick adobe walls and inner courtyards with fountains are used, cooling the air through water evaporation and locally increasing humidity to comfortable levels.

Interesting fact: Air conditioning efficiency is evaluated not only by temperature but also by humidity. Modern systems "dry" the air by condensing excess moisture on cold evaporators. However, excessive drying (below 40%) indoors is also harmful. Therefore, the "felt temperature" index, which considers both temperature and humidity, is a more accurate measure of comfort.

Favorable Winter Regime: Minimizing Heat Loss and Preserving Moisture

In winter, especially in cold climates with heating seasons, the body faces the opposite task — retaining heat. Meanwhile, air in heated rooms becomes extremely dry.

Indoor air temperature:

Living rooms: 20-22°C. This is the WHO recommended range for healthy adults. At this temperature in typical home clothing (long pants, sweater), heat loss balances heat production at rest.

Bedroom: 18-20°C. Lower temperatures promote melatonin production and deeper sleep, as the body naturally lowers internal temperature slightly at night.

Children's room: 20-22°C for infants, who have more difficulty regulating temperature, and 18-20°C for children over one year old.

Indoor relative humidity: 40-60% remains optimal in winter but is very hard to achieve.

Reality of the heating season: Humidity in apartments often falls to 15-25%. This dries mucous membranes (nose, throat, eyes), reducing their barrier function, makes skin dry, and increases static electricity. Dry air subjectively feels colder because evaporation of moisture from the skin intensifies.

Solution: Mandatory use of humidifiers or alternative methods (containers with water on radiators, wet towels, houseplants). Ventilation in winter, although it lowers temperature, almost does not increase humidity because cold outdoor air contains little water vapor.

Practical example: Finnish and Swedish homes, known for their energy efficiency, pay great attention to supply and exhaust ventilation systems with heat and moisture recovery. This allows retaining up to 90% of heat and maintaining humidity at comfortable levels (40-50%) even in harsh winters, without window fogging or stuffiness.

Outdoor Conditions and Acclimatization

Favorable outdoor parameters depend on acclimatization. A Siberian resident will feel comfortable at -10°C in dry, windless weather due to adaptive reactions (peripheral vessel constriction, increased basal metabolism). For a resident of Sochi, this would be extreme cold. Wind (wind chill effect) sharply increases heat loss by convection, shifting subjective comfort perception toward higher temperatures.

Harmful Extremes: Overheating and Hypothermia

Overheating (hyperthermia): Occurs when body temperature exceeds 38°C. When combined with high temperature (above 32°C) and humidity (above 70%), the risk of heat stroke sharply increases. Especially dangerous for children and elderly people whose thermoregulation system is less effective.

Hypothermia: Begins when internal temperature falls below 35°C. Humidity worsens the situation because wet clothing loses insulating properties and sharply increases heat loss.

Practical Recommendations for Maintaining Comfort

Summer: Use air conditioning to maintain 24-26°C and 40-50% humidity. Ventilate actively at night and in the morning. Wear clothing made of natural, hygroscopic fabrics (cotton, linen) that do not hinder evaporation.

Winter indoors: Control temperature with a thermostat, avoid overheating (above 23°C). Mandatory use of humidifiers to maintain 40-50% humidity. Ventilate briefly but intensively.

Outdoors: Dress according to weather, considering humidity and wind. Damp cold requires more serious insulation than dry frost. In heat with high humidity, minimize physical activity and increase water intake.

Conclusion: Dynamic Balance as the Key to Health

Favorable temperature and humidity conditions for humans are not static numbers but a zone of physiological and psychological optimum depending on season, activity, clothing, and individual adaptation. The key principle is balance. In summer, this balance is achieved through effective evaporation; in winter, through retaining heat and moisture in the body and living environment. Understanding these mechanisms allows not only comfortable existence but also reducing cardiovascular strain, supporting local mucosal immunity, and increasing overall resilience to environmental stresses. Ultimately, creating the right microclimate is an investment in health, productivity, and quality of life in any season.

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