ISO 15714: 2019. Method of evaluating the UV dose to airborne microorganisms or viruses transiting in-duct ultraviolet germicidal irradiation devices (UVGI). 

Test not accredited in our laboratory.

Microorganisms (bacteria, fungi or their spores) and viruses can be present in the air and some can cause illness when breathed in with the air. For this reason, procedures have been developed to disinfect the air and inactivate microorganisms or viruses, or their nucleic acids (DNA or RNA) that may be present in it, before being introduced inside homes or comercial premises, with the aim of prevent airborne infections. One of the procedures uses devices that generate ultraviolet germicidal radiation (UVGI: Ultraviolet Germicidal Irradiation) with a wavelength between 240 and 280 nm. In these UVGI devices the disinfection of the air is achieved by means of UV lamps mounted in the ducts of the heating, ventilation and air conditioning (HVAC: Heating, Ventilating and Aire-Conditioning) systems. A UVGI device consists of a filter through which new air penetrates, UV lamps, followed by a heating or cooling coil system that is then distributed through the air conditioning outlets.

To evaluate the activity of a UVGI device, the ISO 15714: 2019 standard describes a test equipment, designed to simulate the real conditions of use in the ducts of HVAC systems, and even the usual standard microorganisms that can be used as generic representatives of the different sensitivities to UV radiation. Additionally, it sets out how to perform the UVGI device evaluation process. With this test, UVGI devices can be evaluated based on air flow velocity and allows reliable evaluation and comparison of the inactivation capacity of microorganisms and the performance of UVGI devices.

In the tests, it is recommended to use surrogate microorganisms as representatives of some of the pathogens for which it is desired to demonstrate the disinfecting efficacy of the UVGI device, in order to make its handling safer than the real pathogen, avoiding the risks of causing infections in the personnel conducting the tests in the laboratory. The standard recommends Serratia marcescens as representative test microorganisms of bacteria with high susceptibility to UV radiation, such as gram-negative bacteria (to test UVGI devices with an effective UV dose of less than 25 J/m2); Bacillus subtilis, as a representative of gram-positive and spore-forming bacteria with low susceptibility to UV radiation (for testing UVGI devices with an effective UV dose of 25 J/m2 to 120 J/m2); and Cladosporium sphaerospermum as a representative of fungi with high susceptibility to UV radiation (to test UVGI devices with an effective UV dose greater than 120 J/m2). The standard does not indicate the use of viruses, but in our laboratory we can adapt the standard to evaluate the viral inactivation effectiveness of the device using the appropriate surrogate viruses among those requested by the client.

The test device described by the standard consists of a 5-meter-long conduit built with galvanized steel or aluminum, square section, and divided into three parts: a) an initial or upstream part, where the inoculation is carried out (2 m); b) a central part where the UVGI device is located (1 m), and c) an end or downstream part (2 m) where the sampling outlet is located. At the front end there is an air insufflator (blower), an air flow regulator and a HEPA filter to avoid introducing microorganisms other than those used in the test with the air blown. On the other sid, conected with the downstream duct, another HEPA filter to avoid dispersion of the test microorganisms to thev environment.

The test microorganisms are introduced into the test equipment at the concentration recommended by the standard and with the UVGI device turned off their concentration must be checked to use the value obtained as a control of the inactivating effect of the duct. Then, to know the effectiveness of the UVGI device, the measurements are made with the UV lamp turned on and preheated, taking the measurements at the sampling part. Sampling is done in triplicate using an Andersen-type stage impactor. For the sampling measurements to be valid, it must be demonstrated that there are no differences greater than 50% between the different measurements. In addition, the microorganism quantification tests must be performed at three different air flow rates of 1,000 ± 100 m3/h, 2,000 ± 100 m3/h, and 3,000 ± 100 m3/h (total of nine control samples without exposure to the UV radiation and nine other samples with exposure to UV radiation for each microorganism or virus evaluated).

The averages of the counts of the microorganisms present in the air before and after UV irradiation are used to calculate the inactivation rate of the test microorganism for each air flow, obtained with the UVGI device tested (N0/N percentage or its logarithm).

The UV dose (D) is the product of UV irradiation and the exposure time to which a certain microorganism or surface is exposed (mJ/cm2). The UV dose of the UVGI test device can be determined from the inactivation rate and the UV susceptibility constant (k) of the test microorganism.

The UV susceptibility constant (k), indicates the degree to which a microorganism is sensitive to UV light or how easily it can be inactivated by UV radiation. The ISO 15174 standard indicates various k ranges for different test microorganisms obtained from bibliographic references.

From the calculation of the UV dose (D) of the UVGI device for the different air flows tested, the UV dose-air flow rate curve can be determined, which is used as an indication of the effectiveness of the UVGI device. A UVGI device with a higher UV dose under the same airflow will have a greater ability to inactivate microorganisms and disinfect the air.

If the susceptibility constant of the test microorganism is not known, it can be determined using a second experimental device described by the standard. This device consists of an opaque duct with a square section, low reflectivity, which widens in the center to form a UV exposure chamber, equipped with a top panel of transparent silica glass to UV light, in which four UV lamps are placed. The intensity of UV rays in the chamber can be controlled by the number of lamps switched on or by inserting fine wire mesh panels, placed between the lamps and the chamber to reduce the passage of radiation. Air is passed through the device at a certain speed, regulating the flow. Furthermore, you can measure the temperature and humidity at the beginning and end of the test can be measured. Microorganisms are injected into the device at a certain concentration and a sustained air flow. The microorganism, its preparation and the test conditions must be the same as those used in the inactivation test with the UVGI device to be evaluated. Samples of the emitted air, both without radiation, and with five different UV radiation doses are used to determine the inactivation rate (log N0/N). Having measured the administered doses (D), and knowing the inactivation rate, the value of the sensitivity constant (k) can be deduced. Knowing the k value of the test microorganism, the UV dose of the evaluated UGVI device and the UV dose-air flow rate curve can be calculated for the evaluation of the UVGI capacity.