Global temperatures broke records this summer as extreme heat caused by climate change roasted parts of China, the Middle East, and the American Southwest. But humidity also factors into how humans cope with and experience heat as expressed by a measurement called wet bulb temperature.
The temperature we typically see in weather reports is the dry-bulb temperature, a measurement of ambient outdoor air temperature. Wet-bulb temperature, which can be taken with a thermometer wrapped in a wet cloth, shows the extent to which the body can be cooled by evaporation from sweating.
“Drier air can absorb water faster, so lower humidity means lower wet-bulb temperature—your body is able to stay cooler than the ambient temperature through evaporation. At 100 percent humidity, wet-bulb and dry-bulb temperatures are the same because the air can't absorb more water, which means sweating can't cool us down,” said ClimateCheck Head of Data & Risk Visualization Annie Preston.
A similar measurement called wet bulb globe temperature is taken with equipment that measures wind speed and solar radiation as well as temperature to give a more complete picture of how heat stress can affect the human body.
Extreme heat coupled with high humidity can affect human health after just a short period of exposure. High wet-bulb temperatures can affect the habitability of buildings and potentially damage building materials. As periods of extreme heat become more dangerous around the United States, real estate investors must take wet-bulb temperatures into account to ensure the health of building residents and outdoor workers, as well as the resilience of long-term investments.
Risks of Wet-Bulb Temperature for Humans
High heat with humidity poses a more significant risk to human health than high heat alone. The human body relies on the evaporation of sweat to cool itself, but in extreme heat and high humidity, sweat doesn't evaporate effectively. This leads to increased body temperature, heat exhaustion, and potentially fatal heatstroke.
High wet-bulb temperatures can indicate significant health risks for humans, especially vulnerable people like children, the elderly, and those with heart conditions or other chronic illnesses. While a wet-bulb temperature of around 95 degrees Fahrenheit is the theoretical limit for human survival, more recent research indicates that in practice, a wet bulb temperature of 88 degrees Fahrenheit can be hazardous even for young, healthy people.
“At high enough wet-bulb temperatures —approaching 95 degrees Fahrenheit — we can't survive for long, even with fans and shade. In these conditions, we rely on air conditioning — and the electricity to power it — to live,” said Preston.
Damp areas in buildings such as basements or other areas exposed to high humidity for long periods can also create health hazards for residents. Exposure to damp environments can cause respiratory issues like shortness of breath, coughing, wheezing, and asthma development in children, according to the EPA. Those with preexisting respiratory conditions are particularly vulnerable. People who are immunocompromised are at higher risk of contracting fungal diseases and other opportunistic infections when exposed to damp indoor environments. Efficient, functional air conditioning can keep people cool and reduce dampness in buildings, even with very high heat and humidity levels.
How High Wet-Bulb Temperatures Can Affect Buildings
The conditions that create hazardous wet-bulb temperatures can affect buildings as well as their residents. Moisture is the biggest threat to the longevity and durability of residential buildings, according to the U.S. Department of Housing and Urban Development. It can contribute to the degradation of building materials like wood and metal, affect insulation and paint, damage furnishings, and promote the growth of hazardous contaminants like mold and mildew. The dehumidifying power of a functional HVAC system can help prevent structural damage while protecting resident health.
High heat and humidity can reduce the efficiency of air conditioning units, which dehumidify buildings while cooling indoor air. In high-humidity conditions, an air conditioner must work harder because the air's ability to absorb excess moisture is reduced. Dehumidifiers in basements or other damp areas will also have to work harder to remove moisture.
“These conditions can mean higher energy use and costs, potentially shortening the lives of appliances, and increased consequences in the case of a power failure,” said Preston.
Sizing HVAC systems appropriately for a given building based on American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standards can help ensure better performance and efficiency.
In hot regions with low humidity, swamp coolers or evaporative coolers can be an effective and energy-efficient way to cool a space with minimal energy use. Fans pull outdoor air into the unit and over a water-saturated pad. This method can lower indoor temperatures by 15 to 40 degrees Fahrenheit. However, swamp coolers can become ineffective at higher wet-bulb temperatures, as the air can’t hold additional moisture from evaporative cooling.
High wet-bulb temperatures is a serious consideration that threatens human health and building materials. ClimateCheck’s climate risk assessments for locations in the US and Canada project both wet-bulb and dry-bulb heat risk. Our assessments also project risk of flood, extreme precipitation, wildfire, and high winds. Our clear and comprehensive climate risk reporting gives investors the power to make smart decisions about their investments.
Alexandra Jones is a Philadelphia-based writer and author who covers agriculture and climate.