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Air quality control and the spread of tuberculosis

Clean air purifier systems avoid disease

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Air in buildings often contains potentially health threatening bacteria and viruses, particularly for people who have impaired immune systems. To reduce the risk of TB and other diseases, air quality control is vital.

South Africa is one of the countries with the highest burden of tuberculosis (TB), with the World Health Organisation (WHO) statistics giving an estimated incidence of 500 000 cases of active TB in 2011.

An estimated 1% of the population of approximately 50 million develop active TB each year. This is the third highest incidence of any country after India and China and the incidence has increased by 400% over the past 15 years according to WHO figures.

Pulmonary TB is caused by the bacteria Mycobacterium tuberculosis (M. tuberculosis). You can get TB by breathing in air droplets from a cough or sneeze of an infected person.

The resulting lung infection is called primary TB. People who have TB typically display symptoms such as a persistent cough, fatigue, weight loss, a fever, coughing up blood and night sweats.

Air in buildings often contains potentially health threatening bacteria and viruses, particularly for people who have impaired immune systems. TB is spread when a person who has tuberculosis coughs or sneezes, thereby releasing the bacteria into the air in the form of an aerosol.

Outbreaks of tuberculosis have been reported in homeless shelters, prisons, health care clinics, school, and recently in call centres and banks.

To reduce the risk of transmission of disease, indoor air quality can be controlled in three ways: dilution, filtration and purification by ultraviolet germicidal irradiation (UVGI).

In addition to controlling tuberculosis, these solutions are also effective at controlling other microbial disorders such as influenza.

Air Dilution

Dilution reduces the concentration of infectious agents in a space by increasing the amount of outside air brought into that space. Dilution does not destroy the bacteria, but 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. 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 by the heating or cooling necessary to ensure thermal comfort.

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

Air 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), 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, so there is 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.

Air Purification

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. 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.

UVGI technology has long been used in laboratories and healthcare facilities, but it is also applicable for use in spaces where people congregate. These three approaches can be used separately or in combination.

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 254nm, a wavelength that causes DNA damage to bacteria and viruses. The lamp also emits some visible short wavelengths that appear as blue light. UVGI lamps are based on conventional fluorescent lamp technology except they have a special glass to emit UV and have 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. Upper room UVGI is likely to become a more common feature in buildings and institutions. Upper room UVGI is an effective method for cleansing the air of many types of viruses and bacteria. The technology is well-developed.

It can be easily retrofitted in many buildings. It has been shown to be effective in the laboratory and in this country, still undergoing extensive testing of its effectiveness for preventing the spread of tuberculosis in representative environments.