**5. Modern HVAC filters**

The type of air filter is the first factor people take into consideration before deciding on which air purifier to purchase.

Air filters and electrostatic filter cleaners are typically rated according to the minimum efficiency reporting value, commonly known as the MERV rating. The MERV scale is a measurement scale developed in 1987 by ASHRAE to rate the effectiveness of air filters, which determines efficiency in terms of the size of particle that the filter will capture. Values vary from 1 to 16, with a higher number indicating the greater efficiency of the filter in trapping airborne particles.

#### **5.1 UV lights and filtration systems for cleaner indoor air**

Recently, UV lights have been widely employed in the ducts of HVAC filtration systems. These lights facilitate effective and inexpensive control and solve the

#### *Low-temperature Technologies*

problem of microbial outgrowth in HVAC systems, eliminating up to 99.9% of the microorganisms and destroying airborne viruses, bacteria, and fungi. The types and quantities of microorganisms killed depend on the length of exposure and the output of the lamps. Nowadays, more advanced UV lights, such as air scrubbers, are employed, which can kill both airborne viruses and bacteria and those growing on surfaces.

There are two main types of UV lights used for HVAC systems, the most common of which are coil sterilization UV lights, which are installed near the return ducts, so as to kill mold that may grow on the air handler coil. These UV lights operate 24 h a day and eliminate the need for removing mold from the air handler coils. The second main type of UV lights is air sterilizer UV lights that function by sterilizing the air passing through the return ducts [24].
