Volatile Organic Compounds (VOCs) are a diverse group of chemicals that readily become gases or vapors at ordinary room temperatures. They are common indoor air pollutants, often emitted from household items, building materials, and people. While some VOCs are odorless or have a pleasant scent, many are linked to adverse health effects, including irritation and long-term respiratory issues. Since most people spend roughly 90% of their time indoors, air quality is important, especially during sleep. Scientists frequently observe that VOC concentrations peak inside homes, particularly in bedrooms, during nighttime hours. This spike results from three distinct mechanisms converging in a sealed environment.
Reduced Air Exchange and Ventilation
The primary physical cause for VOC concentration increases at night is a significant reduction in the air exchange rate (AER) of the living space. Air exchange is defined by the number of times the total air volume in a room is replaced with fresh outdoor air in an hour, measured in air changes per hour (ACH). During the daytime, activities like opening windows, frequently opening and closing exterior doors, and mechanical ventilation systems running at higher speeds all contribute to a relatively high ACH. This constant influx of fresh air effectively dilutes any pollutants, keeping the concentration of indoor VOCs low.
The situation changes drastically as occupants prepare for sleep and during the night. For reasons of comfort, security, and energy conservation, people tend to seal their homes by closing windows and doors, and often reducing the fan speed or turning off the mechanical heating, ventilation, and air conditioning (HVAC) system. This behavioral change drops the ACH significantly, often to typical residential rates between 0.2 and 1.0 air changes per hour. When this happens, the room behaves like a sealed container where new emissions have nowhere to go.
A continuous source of VOCs, whether from a piece of furniture or a person, will lead to a rapid buildup in concentration when the dilution rate is low. When the home is sealed at night, the VOCs emitted throughout the night accumulate, causing the observed concentration spike until the room is ventilated again.
How Temperature and Humidity Affect Emission Rates
Beyond the physical reduction in air exchange, chemical and physical processes related to the indoor environment accelerate the release of VOCs from materials. This process, known as off-gassing, is strongly influenced by both temperature and relative humidity (RH).
Temperature Effects
The emission rate of VOCs from solid materials, such as paint, furniture, and carpets, is directly proportional to the ambient temperature. Higher temperatures increase the kinetic energy of the chemical molecules embedded in these materials, making it easier for them to break their bonds and diffuse into the air. Studies have shown that a temperature increase can significantly increase VOC concentrations, depending on the material. Even if the indoor temperature drops slightly at night, the cumulative effect of a sealed space trapping the off-gassed chemicals is what drives the concentration spike.
Humidity Effects
Relative humidity also plays a significant role in off-gassing, especially for water-soluble VOCs like formaldehyde. High humidity mobilizes these chemicals, increasing their emission rate from materials such as pressed wood and drywall. Since occupants in a sealed bedroom exhale moisture, the relative humidity in that space often rises during the night. This combination of reduced ventilation and increased RH acts as a powerful catalyst, driving a heightened release of pollutants from the surrounding materials.
Occupant-Generated VOCs and Evening Activities
A direct source of nighttime VOCs is the occupant of the room themselves, contributing what are known as bio-effluents. Humans continuously exhale various VOCs as metabolic byproducts, with compounds like isoprene and acetone being among the most abundant released in breath. These emissions build up quickly because the sleeping occupant is confined to a small, poorly ventilated space for an extended period.
In addition to breath, the human body sheds VOCs from skin oils through a process called skin oil oxidation. When a person is stationary under bedding, these skin-based emissions are concentrated near the body and become a continuous source of pollution. The concentration of nearly 100 different VOCs has been observed to be high in bedrooms during sleeping periods compared to other rooms in the same residence.
Furthermore, evening routines just before sleep introduce a burst of VOCs that become trapped in the indoor air. Activities such as cooking dinner, showering with scented products, and using personal care items like hairspray, perfume, or lotions release a large volume of chemicals. This significant initial emission, combined with the immediately following period of low ventilation and continuous human off-gassing, sets the stage for the peak VOC concentrations observed upon waking.