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HVAC/R Equipment Design and Worker Safety in the Use of UV-C Germicidal Irradiation

Learn about technology that deactivates biological growth on surfaces within an air handler to save energy, boost airflow levels and protect indoor air quality.


June 4, 2020

Since the 1990s, ultraviolet germicidal irradiation (UVGI), or light in the UV-C wave­length (specifically 253.7 nm), has been used in HVAC/R equipment to maintain or return cooling capacity to “as-built” conditions by maximizing heat exchange efficiency. The technology deactivates biological growth on surfaces within an air handler to save energy, boost airflow levels and protect indoor air quality.

 ASHRAE Technical Committee (TC) 2.9, Ultraviolet Air and Surface Treatment, helps oversee the safety stan­dards that protect installers and service personnel from accidental UV-C exposure. Complementary HVAC/R equipment requirements in this area are published in the UL 60335-2-40 safety standard for heating and cooling equipment.

Limiting UV-C Exposure

Ultraviolet light is separated into three main seg­ments along the electromagnetic spectrum. The most well-known wavelengths are UV-A and UV-B, which are responsible for sunburns. UV-C is also produced by the sun but cannot penetrate the atmosphere due to its shorter wavelength. When generated by artificial sources, however, close proximity to UV-C irradiation is powerful enough to break down the skin’s collagen pro­teins, causing redness and irritation. It can also damage the surface layer of the cornea in the eye, resulting in photokeratitis (the same condition welders can experi­ence with arc flashes).

Not all HVAC/R technicians receive UV-C safety train­ing and some may not be aware that the UV-C light in an air-handling unit (AHU) can potentially pose a safety hazard. Unlike some dangers, exposure to ultraviolet light does not offer an avoidance response (e.g., blink­ing of eyes) or a physical cue that protection is necessary (e.g., heat radiating from a hot stove). Furthermore, the physiological effects of an adverse dosage of UV-C expo­sure are delayed and can appear up to six hours later. While damage from UV-C is reversible, the HVAC/R industry takes steps to safeguard service personnel from avoidable ultraviolet exposure and the consequences of its short-term or chronic effects.

Many HVAC/R and UV-C equipment manufacturers have voluntarily implemented safeguards against the risks of UV-C exposure. Instructions and signage advise service personnel that the UV system should be turned off before performing any work in the AHU. A mainte­nance worker can easily take this step before opening the AHU to service the motor, change fan belts, replace filters, check coils, or drain pans. Some manufacturers include a door safety switch or lockout/tagout feature to keep the AHU closed until the UV lamp power has been disconnected.

Despite these precautions, reports from the field indi­cate that safeguards are not uniformly implemented or equally effective. Even if workers had been adequately briefed about UV radiation, UL recognized that by standardizing equipment design safeguards, the risk to installers, service per­sonnel, and facility managers could be greatly reduced.

By implementing safety features within the AHU, UL60335-2-40, third edition, standard seeks to eliminate variations in equipment design that might lead to acci­dental exposure. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) favors this approach: “engineering control measures are prefer­able to protective clothing, goggles and procedural safety measures.” In other words, deliberately designing-for-safety to protect workers from potential hazards is more important than supplemental warning systems.

A Look at the Equipment Design Requirements

The safety requirements in UL 60335-2-40 apply to UV lamps that are factory and field installed for HVAC/R equip­ment manufactured after Nov. 2019. These updated requirements will help ensure that the risk to service workers is reduced regardless of their proximity to the UV-C source. Requirement highlights are as follows:

Permitted Irradiance Levels

The primary source of UV used for inactivation of bio­logical growth is low pressure mercury lamps. The lamp construction emits most of its UV at 254 nanometers (nm). Therefore, the irradiance levels from HVAC/R equipment sources were chosen based on the thresh­old limit value (TLV) of 6.0 microwatts per centimeter squared (μW/cm2) of weighted UV at 254 nm.2 The ultra­violet  emissions are measured at the nearest point of accessibility.

  • No opening shall permit leakage of UV-C greater than 0.1 microwatts per centimeter squared (μW/cm2).
  • Enclosure doors, covers, and access panels, when accessed, shall have no UV-C irradiance greater than 1.7 μW /cm2.
  • The occupancy or space around the HVAC&R equip­ment shall not exceed 0.1 μW/cm2.
  • Adjoining compartments inside the unit without an interlocking switch shall be no greater than 1.7 μW /cm2.

Enclosure Doors, Covers, and Access Panels

  • These points of intentional access to UV sources must be equipped with an interlocking mechanism that de-energize the source.
  • Access panels and components that are removable for cleaning and servicing, which also provide access to other areas inside a unit, shall require an interlock switch if the UV-C irradiance exceeds 1.7 μW /cm2.

Equipment Markings

  • Models with interlocking doors shall be marked with the words “WARNING UV LIGHT SOURCE.”
  • Those without interlocking doors shall have the same wording plus “DISCONNECT POWER BEFORE SERVICING.”

In addition, the equipment shall be plainly marked, in a permanent manner, with the following:

  • Details on the UV source manufacturer’s name and model designations that can be installed in the AHU. The details can be provided as a label that allows the AHU manufacturer to “check off” the lamp system installed.
  • The ratings of the system lamps, maximum lamp rating in watts, and voltage of the unit with the ultra­ workers is reduced irrespective of their proximity to the UV-C source. Highlights of the new requirements are as follows:

Housing Materials

  • Polymeric cabinet, structural and current carrying parts and wiring that are subjected to irradiance from a UV lamp system shall be shielded from the UV-C light or constructed of a material that is capable of withstanding UV-C exposure levels expected in the product without degrading.

The standard requirements are in line with OSHA 1910.1096 for ionizing radiation. The government agency for workplace safety advocates for reasonable protections, including safety briefings, caution signs, personal pro­tective equipment (PPE) and “a control device which shall either cause the level of radiation to be reduced or shall energize a conspicuous visible or audible alarm signal in such a manner that the individual entering and the employer or a supervisor of the activity are made aware of the entry.”

Moving Forward

When certifying HVAC/R equipment to UL 60335-2-40, AHU manufactur­ers will need to perform the irradiance test on their equipment using a qualified Nationally Recognized Testing Laboratory (NRTL). (UL has such laboratories.) In-house testing by the AHU manufac­turer can also be done if it is qualified to test to UL 60335-2-40 or if the test is conducted while being witnessed by a qualified NRTL representative. The test must be done in a controlled environment with an ambient tempera­ture of approximately 80°F (27°C) using either a scan­ning spectroradiometer or a narrow band range radi­ometer if the UV wavelength is known (i.e., 254 nm). The test should also be conducted with all components opened to the widest degree possible so measurements represent any point/degree of exposure within the ser­vice area.

The standard is intended for type-certification and does not require companies to test every unit that is shipped. Manufacturers can choose to certify a repre­sentative AHU “box.” The box will be tested for the maxi­mum amount of UV energy that it can safely accommo­date (total of the cumulative lamp watts printed on the lamp surface, e.g., 145 W). Should the manufacturer use a lower cumulative UV-C lamp wattage on future pro­duction units than the representative tested unit, testing will not be necessary.

UL 60335-2-40 distinguishes between UV-C lamps located within the AHU and duct-mounted or non-integral field-installed systems, which are predominantly found in residential applications. Though the require­ments are substantially the same, UL 60335-2-40 offers “Supplement SA” that requires the use of many of the same safety precautions. For the purposes of this stan­dard, UV-C lamps are limited to the low pressure fluo­rescent type construction with a predominate output of 254 nm.

The Value of UV-C

UV-C is a low-cost solution to disinfect cooling coils, drain pans and duct surfaces that have accumulated mold and bacteria growth. The technology disrupts a microorganism’s DNA, triggering a chain reaction that leads to cellular death. Because the lamps operate con­tinuously, biofilms are unable to regenerate, provided the UV-C source is properly maintained. This technol­ogy is frequently used to address many sources of poor indoor air quality that contribute to employee discom­fort and absenteeism.

Contaminants, particularly the presence of fungi (mycotoxins), can trigger serious health problems to building occupants. As noted in an Applied and Environmental Microbiology study, “fungi have been found growing on air filters, insulation and cooling coils, as well as in ducts. This contamination often contributes to building-related diseases, including both infectious diseases and hypersensitivity diseases, such as allergic rhinitis, asthma and hypersensitivity pneumonitis. Also, acute toxicosis and cancer have been attributed to respi­ratory exposure to mycotoxins.”  A building’s HVAC sys­tem can also inadvertently transmit rhinoviruses (com­mon cold), tuberculosis, measles, SARS, influenza and other airborne microorganisms.

“The effectiveness of a UV-C system to inactivate microorganisms in the air and/or on surfaces has been sufficiently demonstrated; the best results were obtained for the long-term irradiation of downstream coil surfaces to avoid fungal amplification on wet surfaces,” notes a 2015 ASHRAE Position Document on Filtration and Air Cleaning.

As an added value, its ability to constantly clean the interior workings of the AHU can extend the equip­ment’s life for prolonged savings. Biofilms on coil fins adversely affect heat transfer to/from the airstream. If mechanical cleanings are incomplete or ignored, up to 25% of cooling capacity can be lost in as little as five years. Another factor is the lack of personnel or labor hours to routinely address coil maintenance. UV-C sources help restore an AHU to its original operating capacity.

The HVAC application of UV-C is nearly universal, including offices, schools, hospitals, airports, correctional facilities, laboratories and assisted living. UV-C sources represent a small investment (roughly 3%) relative to the overall cost of the AHU, or roughly $0.15 per cfm, and are easy to retrofit. Building own­ers can achieve a 10% to 25% increase in HVAC effi­ciency by adding a UV-C device. The safety features in UL60335-2-40 will help ensure service personnel are protected from accidental exposure, while maintaining the effectiveness of UV-C to eradi­cate biological contaminants.


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