Indoor Air Cartoon Journal, March 2019, Volume 2, #48

Liquid droplets (liquid particles) are released into the air when someone coughs, sneezes, exhales, or talks. The droplets may exist alone or attach themselves to existing particles in the indoor environment. Coughing and sneezing increase the intensity and amount of the droplets ejected. The extent of the adverse health effects to indoor occupants depends on the toxicity of the viable infectious organisms the droplets are carrying. An infected person will eject more harmful organisms. Some of the viable infectious organisms are life threatened even at low concentration and short exposure while others require a high concentration and extended exposure duration to be life threatened.

The droplets are in different sizes. Large droplets deposit easily on indoor surfaces. Small droplets, like those in the PM2.5 size range, linger in the air because their mass is lower than that of large size, e.g., PM10, droplets. The lower mass of the small size particles causes the drag force moving them in the air to be higher than the gravitational force enhancing their surface deposition. Exposure to the organisms may occur when a non-infected person is present in the indoor environment – building, bus, train, etc. – with an infected person ejecting the infectious droplets.

Exposure may also occur when a non-infected person enters an indoor environment long after the infected person has left the indoor environment. This is more likely in the case of small size particles. The probability of inhaling large size particles long after ejection or the infected person has left the indoor environment is much lower than that of small size particles. The size of the particles determines where they cause harm to human. The small size of the particles makes it more natural for organisms associated with them to penetrate deep into the lungs, and even blood. Large size particles are effectively filtered at the upper respiratory tracts, and their health effects are predominate at this region.

Ventilation, filtration, and air cleaning through inactivation of the viable infectious organism are used to reduce the concentration of the airborne organisms. In as much as the increase in ventilation rate could reduce the concentration of the organisms, the cost – energy consumption – of using it effectively may be too expensive. Air filtration is used to reduce reliance on ventilation and save energy. Making the viable infectious organisms inactive with the use of ultraviolet germicidal irradiation (UGVI) has proofed to be very effective and less energy intensive. Unlike ventilation and air filtration, UVGI is a specific solution for reducing concentration and exposure to viable infectious organisms. Its usage is popular in healthcare facilities but can be used in other indoor environments.

Do you want to know more about the airborne transmission of viable infectious organisms in the indoor environment and energy efficient method of effectively reducing their concentration? Read Mphaphlele et al. (2015) and Narden et al. (2008) papers.