The COVID-19 pandemic has raised awareness about the topic of indoor air quality (IAQ), including aspects related to pathogens like the SARS-CoV-2 virus, and other contaminants such as dust, allergens, volatile organic compounds, etc. While there are numerous indoor air quality products available in the market, not all products utilize the same technology or deliver the same range of performance. To identify a product that is safe, effective, and suitable for the transit environment, it is important to understand the specific CDC and ASHRAE guidelines regarding indoor air quality technologies.

Proven Effectiveness

The CDC recommends that in selecting an indoor air quality product, consumers should request testing data that quantitatively demonstrates a clear protective benefit and occupant safety under conditions that are consistent with the intended use. It is also recommended that this data be from independent, third-party sources1. In keeping with CDC guidance, Thermo King®, a strategic brand of Trane Technologies, devoted significant resources to conduct expansive comparative in-chamber testing in February 2021 at an independent laboratory. The testing consisted of a modeled transit bus cabin and simulated airflows, which enabled a fact-based evaluation of IAQ technologies for transit bus HVAC applications. Representative airflow, testing chamber size, and scaling of the air quality device are critical to understanding how the technology functions in the desired application. Successful modeling cannot be replicated accurately in small test chambers. Through this independent third-party testing, we were able to select and validate our air purification solution as safe, effective, and suitable for the transit environment. Further, to approximate real life situations even more closely, Thermo King subsequently commissioned independent third-party tests for our Air Purification Solution in an actual transit bus. The in-bus tests were conducted in May 2021 by the same independent third party as the original in-chamber tests, validating the test results.

Safety, Efficacy, and Suitability

Aside from testing, there are also numerous safety factors to consider when evaluating IAQ technologies. For the transit environment, buses are occupied spaces and often densely filled. IAQ technologies should not have side effects that could introduce new or increased safety risks for passengers and drivers. For instance, some products use UV light at wavelengths and power levels that can cause harm to humans.  Other UV technologies use mercury vapor lamps to emit UV light, which have the risk of fracturing and leaking potentially dangerous levels of mercury into the passenger area.

Thermo King’s Air Purification Solution utilizes a UV-A LED light array that produces a wavelength that is much safer for humans (e.g., skin and eyes) and for materials (e.g., plastics) compared with a UV-C light. Experts also note that if UV-C light is being used, it is important to ensure that it is properly contained, and humans are not exposed to it. Thermo King’s UV-A LED light array is mounted behind the return air grill and under a filter.  It is also directed away from the passenger area of the transit bus. Furthermore, the use of LED lights helps protect passengers from safety risks in case of light damage, and offer extended component life spans, lasting as much as five times longer than other solutions.  

Another side effect of some IAQ technologies to consider is the generation of ozone. Ozone is a reactive gas that can disinfect air and surfaces, however, ozone can be harmful to humans and exposure to ozone creates risk for a variety of symptoms and diseases associated with the respiratory tract2. Due to the risks associated with ozone exposure, the CDC cautions that ozone should never be used for disinfection in occupied spaces. To ensure the selection of an air disinfection device that will not expose passengers to ozone, the CDC recommends air disinfection devices that comply with the UL 2998 standard certification, which validates that no ozone is produced3. Additionally, ASHRAE (The American Society of Heating, Refrigerating, and Air-conditioning Engineers) Standard 62.1-2019 Ventilation for Acceptable Indoor Air Quality requires that air-cleaning devices comply with the UL Standard 2998 and that air cleaning devices shall be listed and labeled in accordance with such.

Thermo King’s Air Purification Solution is UL 2998 certified, meaning it does not generate ozone, a common byproduct of many air purifiers. What’s more, Thermo King’s Air Purification Solution oxidizes volatile organic compounds (VOCs), which can potentially be harmful to passengers, helping to create a more pleasant and comfortable passenger experience.

How It Works

The Thermo King Air Purification Solution is a graphene-enhanced photo-catalytic oxidation device that integrates directly with the HVAC system. By directing UV-A light onto a graphene catalyst, it produces hydroxyls, present for nanoseconds, that bond to and destroy contaminants as they pass through the HVAC system. The clean air is then supplied back into the cabin area, forcing contaminated air to circulate back through the bus’s HVAC system for further cleaning. As a result of this process there are no lingering after-effects on surfaces, such as plastics, metals, and rubberized material. When combined with a MERV 7 pre-filter, our active solution has been proven to be 98 percent effective against in-air viruses* within the first 20 minutes of the device being turned on4. As a result, the Thermo King Air Purification Solution is an effective active solution within the high airflow applications of a transit bus HVAC system.



Public bus transit is a major driver of economic stability in the U.S., providing greater access to jobs, schools, and supporting economic mobility. Public transport also supports more sustainable cities. According to the American Public Transportation Association, at the height of the pandemic in 2020, some cities experienced a nearly 90 percent decline in bus ridership.  Thermo King, a leader in sustainable transport solutions, committed to identifying a technology to help riders and drivers feel safer while getting back to their normal routines and using public transportation. Improving the Indoor Air Quality within a bus is one of the first steps. Thermo King’s Air Purification Solution helps drivers and passengers of public transportation feel safer through a proven solution, tested and validated by third party experts.

Air Quality within a bus is one of the first steps. Thermo King’s Air Purification Solution helps drivers and passengers of public transportation feel safer through a proven solution, tested and validated by third party experts. Learn more at: thermoking.com/purification


1 See https://www.cdc.gov/coronavirus/2019-ncov/community/general-business-faq.html#Critical-Infrastructure8-question.
See https://www.ashrae.org/technical-resources/filtration-disinfection#mechanical
See https://www.cdc.gov/coronavirus/2019-ncov/community/general-business-faq.html#Critical-Infrastructure8-question.
4 *While the devices were not tested specifically against COVID-19, the testing results were obtained using MS2 Bacteriophage, a common surrogate for SARS-CoV-2, the virus causing COVID-19. Furthermore, air-cleaning technologies do not replace Personal Protective Equipment and do not protect riders from other riders and should be used in conjunction with guidelines from the U.S. Centers for Disease Control and Prevention (CDC) and other health authorities such as face coverings and social distancing.