Guest Lecture 01- Ultraviolet Germicidal Irradiation

The Ultraviolet germicidal irradiation (UVGI) is a method to sterilize which uses ultraviolet light at suitably short wavelength to fracture the microorganisms. It is mostly used in a different range of purpose like cooking, water purification and air. UV is a famous method since 120 years ago. But the fact is UVGI is dangerous to use and it can damage the structure of the existence in the micro organic level. For example using the UVGI machine in some location like circulating air or water systems makes a poisonous effect on micro-organisms.

Indoor microorganisms are in two different categories. First, Pathogens or more commonly germ which is a biological mediator that causes illness to its mass. It includes bacteria and virus. In fact, our body develops numerous natural orders of defense against some of the common pathogens but cannot cover all of them. Pathogenic viruses cause different types of illness like chickenpox, measles, smallpox and rubella. The range of the virus is between 20-300 nanometers in length and it is really small compare to the range of bacteria which is between 1-5 micrometers in length. Second category is Fungi which is includes a eukaryotic territory of microbes that are generally saprophytes but mostly can develop infection in people, animals and plants. Fungi are usually making a yeast infection in the body like in skin and nail. It is usually grow in presence of food like organic material and water. In HVAC system mostly found in cooling coil and damp filter media.

The big discussion in indoor microorganisms is how to control it. As we know HVAC system (Heating, ventilation and air-conditioning) can participate in distribution and control of transmittable microorganisms. For example, for Pathogens the best way to control is remove it from air, remove from surface and limit person to person transmittal with geo hygiene. In addition, for fungi the best answer is control the surface moisture. HVAC ducts are treating as a pipe in the building and the transmittable microorganisms can travel room to room when people socialize in the room, for instance when they talk, laugh, cough and sneeze. It is important because it can be suspended in the air for several hours.

Ultraviolet germicidal irradiation (UVGI) is a good option with high efficiency filtration system to organize the airborne microorganisms. UVGI system is typically low pressure mercury vapor lamps which use 254 nm UVC radiations produced to interrupt the DNA or RNA of microbes and cause impairing their capability of replication. The microbial response to UVGI is as this formula: S=exp(-KIT). S is the survival fraction of a microbial population. Factor K (cm2/μW-s) is represents the resistance of the species of interest. I germicidal radiation at a flounce rate (W/cm2) and T is the time on the formula. Remember that Air temperature and velocity may vary over a wide range in an HVAC system and make considerable change in, Also I and t factor which are infrequently measure for a design and analyses in an accurate way.

One of the good examples of the system type for UVGI is UV above occupied. It is a zone irradiance circulation air. Which the first cost was $2.5 ft2 and operation cost was $0.13 ft2/ year. More models are self contained, decontamination and coil filter bank disinfection.