There are three main ways to get rid of airborne pathogens in building interiors. Pathogens can be expelled from the building with exhaust fans, they can be captured with high-performance air filters, or they can be killed with air purification methods. ASHRAE strongly recommends the use of Ultraviolet Germicidal Irradiation or UVGI, which is effective against viruses, bacteria and mold spores.
Ultraviolet radiation is capable of destroying the DNA of coronavirus and other pathogens. Unfortunately, this type of radiation is also harmful for humans, and it cannot be used directly on patients. However, UV rays can be used safely to purify the air, water, surfaces and inanimate objects. This technology should be handled by trained technicians, since direct exposure to UV radiation can damage the eyes and skin. A qualified HVAC engineering firm can help you select the best measures to help protect your building from COVID-19.
Improve your indoor air quality with UV purification, high-efficiency filtration and smart ventilation.
NOTE: Air purification with UVGI and other HVAC measures are intended as a complement for the guidelines from health authorities, not a replacement. Social distancing, frequent handwashing and personal protective equipment (PPE) are required even if your building has the best air purification system available.
How UV Radiation Can Kill Pathogens
Ultraviolet radiation has wavelengths that range from 100 to 400 nanometers (nm), and it is classified into three bands depending on the wavelength:
- UV-A, from 315 to 400 nm
- UV-B, from 280 to 315 nm
- UV-C, from 100 to 280 nm
UV-C radiation carries the most energy, and this makes it the most effective for killing germs. According to ASHRAE, the optimal range is 200 to 280 nm, and the highest germicidal effect is achieved at 265 nm. Most UVGI air purification systems use a wavelength of 253.7 nm, which is close to the optimal value.
UV radiation with a wavelength below 200 nm forms ozone, and for this reason it is not recommended for air purification. The ozone in the upper atmosphere protects us from harmful space radiation, but ozone at ground level is considered a pollutant. Ozone causes respiratory irritation and chest pain, and asthma patients are especially vulnerable.
Ozone also has a germicidal effect, but the concentrations required for this are also harmful for humans. For this reason, ozone-based air purification is not recommended for occupied spaces. Before installing an UVGI system, make sure it meets UL Standard 2998 Environmental Claim Validation Procedure (ECVP) for Zero Ozone Emissions from Air Cleaners.
Types of UVGI Systems
There are many types of UVGI systems, which use the same basic principle but different application methods. With a professional inspection, an HVAC engineering firm can help you select the best option for building:
- In-duct air disinfection
- Upper-air disinfection
- In-duct surface disinfection
- Portable room decontamination
In-duct air disinfection consists of using banks of UV lamps inside HVAC equipment or air ducts. These lamps intercept the airflow directly, and they release a high dose of UV-C radiation as the air passes by. According to ASHRAE, they are typically designed for 500 fpm air speed, with a minimum irradiance zone of 2 feet and UV exposure time of 0.25 seconds. The ASHRAE recommendation is using in-duct air disinfection coupled with at least MERV 8 filtration.
Upper-air disinfection is a method typically used for occupied spaces at least 7 feet tall, where the walls and ceilings have a low UV reflectivity. This method is more effective when the ventilation system maximizes air mixing, and additional fans may be needed to achieve enough disinfection.
In-duct air surface disinfection is similar to the first method, but UV lamps are parallel to the airflow instead of perpendicular. Since this layout increases the exposure time, the UV irradiance can be lower. In-duct surface disinfection can also be used for cooling coils, drain pans, and other surfaces that may accumulate moisture.
Portable room decontamination uses a movable device, as its name implies. This method can be easily used for objects and surfaces, unlike the other three. Portable UVGI devices have been used to kill pathogens that are more resilient than coronaviruses.
UV-C radiation is normally produced with low pressure mercury lamps. LED versions are under development, but they are not cost effective at the moment. Consider that LED lighting is normally used to save energy, and UVGI requires a high enough energy output to kill germs. For an equal UVGI output, current LEDs are much more expensive than low pressure mercury lamps. However, this could change in the future.
UVGI has the advantage of dealing with three health hazards at once: viruses, bacteria and fungal growth. The media is currently focused on the 2019 coronavirus (SARS-CoV-2), but Legionella bacteria can also cause severe pneumonia, and they can spread in poorly serviced HVAC installations.