Healthy Indoors Magazine - USA Edition

HI April 2020

Healthy Indoors Magazine

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36 | March 2020 Ultraviolet (UV) Installations in Residential Mechanical Systems ©2020 Jeffrey C. May germicidal effect. (In a Trane's position paper on UV-C on the Trane website, they estimate that a single UV-C lamp in a duct would have to be about a half-mile long to be ef- fective.) It therefore seems misleading that most germicidal lamps for residential use are sold on the premise that they will improve air quality by destroying microbes in the air. The principal effective use of UV-C is surface and not air disinfection, but not all installations further this objective. One of the lamps I saw was installed with the housing per- pendicular to the air flow, preventing both airflow across the lamp and surface irradiation. UV lamp incorrectly installed perpendicular to airflow May Indoor Air Investigations LLC Other UV lamps were far too small, and some were pro- ducing noticeable amounts of ozone gas (which smells like "fresh outdoor air," but which is a pollutant in its own right). M any of you have no doubt seen UV lamps in HVAC systems. There is a lot of hype regarding these lamps, partly because installers and manufacturers often recommend such lamps as a way to improve air quality and to keep a system clean. I am going to play dev- il's advocate regarding use of such lamps in residential me- chanical systems. Let's start with some of the science involved. UV is a part of the continuous electromagnetic energy spec- trum that extends from radio waves and infrared (heat) to visible light, UV, X-rays and gamma rays. UV waves are more energetic than visible light but not necessarily detect- ed by the human eye. UV light has been divided into three categories: UV-A, UV-B and UV-C. Typical black lights produce UV-A, which is not particularly harmful. UV-B is more energetic; these rays (from the sun and sun lamps) cause tanning and are associated with skin cancer. UV-C is more energetic still, and is referred to as "germicidal" because it can destroy the chemical bonds in molecules. UV-C Light as a Disinfectant The idea of using UV-C light to disinfect surfaces and air is not new. Studies were done in the 1930's in classrooms in which shielded UV-C lamps were installed at room ceilings (to avoid human exposure) and found to reduce the spread of German measles. More recent studies have also shown that (shielded) ceiling UV-C lamps can help disinfect the air in hospital rooms. The lamps in HVAC systems are typically the germicidal UV-C type. Are UV-C lamps useful in disinfecting the air moving through a residential HVAC system? The answer to this question is "no," because air moves through the HVAC system too quickly – as quickly as 10 feet per second – too fast for the lamps to have much of a

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