What Does Human Thermal Plume Mean?
The human thermal plume is the constantly rising airflow surrounding the human body, resulting from the temperature gradient between the body's surface and the ambient air.
The human thermal plume can transport particulate matter from the ground into a person's breathing zone, thereby increasing the concentration of inhaled particulates. Studying the thermal plume is essential to finding measures to mitigate exposure to harmful particulate matter, especially in controlled indoor environments.
Safeopedia Explains Human Thermal Plume
The human body generates a convective flow that significantly affects the air distribution and the transmission of contaminants in a room. Studies of the microenvironment around the human body have revealed that there is a layer of air that flows up adjacent to the body surface, starting at foot level. The width of this layer progressively increases as it rises and accelerates. Moreover, manikin studies have revealed that the higher the temperature difference between the surface temperature of the manikin and the ambient air temperature, the faster the airflow around the body.
The human thermal plume also has a major influence on the dispersion and transport of aerosols in the microenvironment. Aerosols are carried into the breathing zone and respirable droplets are intercepted and along the body’s boundary layer. In calm indoor settings, most inhaled air comes from the boundary layer where the human thermal plume flows constantly.
Ventilation and Human Thermal Plume
Controlling the airflow around the human body is necessary to improve thermal comfort and the quality of inhaled air.
The microenvironment around the human body consists of a buoyant convective flow with a thickness of up to 50 mm and a velocity of up to 0.25 m/s. This flow transitions into a thermal plume overhead, and merges with transient flows generated by breathing. The airflow in the microenvironment is an effective transport mechanism for pollutants to travel along the human body up to the breathing zone. It is also an essential element for achieving air movement for displacement ventilation.
In a study on the effects of the human thermal plume on the inhalability of fine/ultrafine particles in stratified indoor air, it was found that for a particle source at floor level and close to the subject, the inhaled particle concentration by the subject is four times higher
than the ambient concentration. The convective heat transfer from the subject induces a buoyancy-driven flow that can transfer pollutants from the occupant’s vicinity to the breathing zone, especially in displacement ventilation.