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Biozone Manufacturing

Indoor transmission of airborne diseases such as COVID19

Airzone indoor air systems protecting against Corona Virus transmission

Photo by Andrea Piacquadio from Pexels

The air in buildings often contains potentially health-threatening bacteria and viruses, particularly for people who have impaired immune systems. The two diseases, i.e. COVID19 or Tuberculosis, are infectious diseases that can be contracted by breathing air containing bacteria or viruses. To reduce the risk of transmission of disease, the air can be disinfected in three ways:

  • Dilution, 
  • Filtration, and 
  • Purification by ultraviolet germicidal irradiation (UVGI). 

In addition to controlling COVID19, these approaches to disinfection are applicable for controlling other microbial disorders such as influenza, tuberculosis and measles.

What technological methods can be used to reduce the risk of indoor infection, and how do they work?

There are three technological methods which can be used to reduce the risk of airborne transmission:

  1. Dilution
  2. Filtration
  3. Purification

Dilution 

Dilution reduces the concentration of infectious agents in a space. This is done by increasing the amount of outside air brought into the occupied portion of that space. It does not destroy the bacteria. It rather reduces the probability of transmission by spreading the bacteria over a larger volume of air. An appropriate level of dilution is achieved by ensuring six air changes per hour in the space. One air change per hour means that the volume of fresh air supplied to the space in one hour is the same as the volume of the space. At six air changes per hour, the air in the space is replaced with fresh air every 10 minutes.

The fresh air required for dilution can be provided by natural or mechanical means. Where natural ventilation is used, additional operating costs may be incurred. This is because of the heating or cooling necessary to ensure thermal comfort. 

Where air conditioning or mechanical ventilation systems are used, dilution requires additional operating costs. It is because of the larger volume of fresh air that must be treated and moved.

Filtration 

Filtration reduces the concentration of infectious agents in a space by passing the air through a high-efficiency particulate air (HEPA) filter that traps bacteria and viruses (and other particles) and thereby removing them from circulation.

Like dilution, HEPA filtration can impose additional operating costs from the increased fan power required to push air through the filter. Few tuberculosis bacteria survive for more than 48 hours on the filter and those that do are difficult to remove and it is, therefore, a minimal risk of re-releasing the bacteria into the air when changing the filter.

HEPA filtration can be used within the ductwork of an air conditioning or mechanical ventilation system, or within a freestanding unit in the occupied space.

Purifying 

Purifying the air through UVGI destroys the infectious agents in the air because exposure to ultraviolet (UV) radiation damages the deoxyribonucleic acid (DNA) of bacteria and viruses, including that of Mycobacterium tuberculosis. This DNA damage stops the infectious agent from replicating. The UVGI technology has long been used in laboratories and healthcare facilities, but it is also applicable for use in spaces where people congregate.

Air cleansing using UVGI requires that persons in the treated space be shielded from excessive exposure to the UV radiation. This can be done by placing the UV source in the ductwork of a ventilation system, in a freestanding disinfecting system or in an open location within a room. 

When installing UVGI in an open location, to prevent undue human exposure to the UV radiation, it is important to ensure that the UV radiation is restricted to the portion of the room that is above standing head height.

These three approaches can be used separately or in combination. The Centers for Disease Control and Prevention (CDC) has recommended that UVGI be used as a supplement to dilution in high-risk settings.

What evidence indicates that these methods are effective?

Dilution, HEPA filtration, and UVGI have all been shown to be effective in reducing the concentration of tuberculosis bacteria in laboratory situations. At the time of this publication, no controlled field studies have been conducted to demonstrate the viability of dilution and HEPA filtration. A multi-city, multi-year study of the effectiveness of air purification through upper room UVGI, called the Tuberculosis Ultraviolet Shelter Study (TUSS), is underway. TUSS seeks to evaluate the effectiveness of upper-room UVGI in homeless shelters as a representative environment of all congregate spaces.

What is the relative effectiveness of dilution, filtration, and UVGI purification?

For air cleansing, the relative effectiveness of dilution, HEPA filtration and upper room UVGI can be measured in two ways. 

One is based on the equivalent air changes per hour; i.e., the number of air changes per hour that would be required to reduce the concentration of tuberculosis bacteria by the same amount as achieved by filtering or upper room UVGI. Using this method of comparison, for dilution at 6 air changes per hour, HEPA filtering provides the level of air cleansing equivalent to 12 air changes per hour. UVGI can provide the level of air cleansing equivalent to 10 to 35 air changes per hour, a range that varies with factors that include UV intensity, time of exposure, and relative humidity.

Another way to compare dilution, HEPA filtering, and upper room UVGI is by their cost-effectiveness. This is expressed in terms of the number of Rands per case of indoor air infection prevented per year. One study estimates that for a high-risk setting (i.e. a hospital waiting room) the cost to avoid a tuberculosis infection was R2 500.00 for UVGI, R8 000.00 for HEPA filtration and R30 000.00 for additional ventilation. Thus UVGI was the most cost-effective of these three technologies.

How is air purification achieved using upper room UVGI?

Upper-room UVGI is achieved by using a UV lamp in a specially designed fixture that directs the UV radiation to the upper-room area.  The UV lamp used for UVGI is a low-pressure mercury discharge lamp. This lamp has a strong emission line at 254 nanometers, a wavelength that causes DNA damage to bacteria and viruses. The lamp also emits some visible short wavelengths that appear as blue light.

For upper room UVGI to be effective, the aerosolised infectious particles must be moved from the lower part of the room, where they are produced by a person coughing or sneezing, to the germicidal zone in the upper-room. 

Practical considerations prohibit the ideal of UVGI cleansing of all infectious particles in one pass when they move through the upper room UVGI zone. The primary consideration is the need to limit the intensity of upper-room irradiance to avoid excessive exposure of humans to UVGI in the occupied part of the room. However, complete inactivation of bacteria and viruses can occur through a cumulative effect of UVGI exposure over time as infectious particles are carried repeatedly through the irradiated upper room. Each pass into the UVGI zone will inactivate a fraction of the infectious particles. This cleansed air further dilutes the concentration of particles in the lower part of the room.

UVGI lamps are based on conventional fluorescent lamp technology except they have a special glass to emit UV. It also has no phosphor coating to produce visible light. Like conventional fluorescent lamps, UVGI lamps are available in linear and compact forms, both of which require ballasts to operate.

The following two videos gives more background.