Air Pollution Filters: Working Mechanism Simplified

Air pollution has become the world's biggest environmental health risk, with around 7 million deaths linked to it in 2012, according to a World Health Organization (WHO) report. The report also highlights a strong correlation between exposure to indoor and outdoor air pollution and cardiovascular diseases, strokes, ischemic heart disease, and cancer. With air pollution posing such a significant threat to human health, it is crucial to understand how air pollution filters work to mitigate these risks. These filters are designed to trap and eliminate harmful pollutants, improving the air quality in both indoor and outdoor environments.

HEPA filters

HEPA stands for "high-efficiency particulate air" (filter). HEPA filters are incredibly effective at capturing almost every size of particle. They can capture viruses, bacteria, pollen, PM2.5, allergens, dust, and more. They are made from either plastic (PP+PET) or fiberglass, and can also capture mould and nanoparticles.

Hydroxyl radicals

Despite their short existence, hydroxyl radicals are extremely chemically reactive. They can effectively eliminate many types of air pollutants, including those emitted by cars, such as nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon monoxide (CO), and particulate matter (PM). When hydroxyl radicals react with these pollutants, they break them down into smaller, less harmful molecules. For example, they can convert NOx into nitric oxide (NO) and nitrogen dioxide (NO2), both of which are less harmful than NOx. They can also react with VOCs, breaking them down into simpler, less harmful compounds. In the case of CO, hydroxyl radicals can oxidize it to form carbon dioxide (CO2). Additionally, they can react with certain components of PM, such as polycyclic aromatic hydrocarbons (PAHs), reducing them into less harmful compounds.

The reactivity of hydroxyl radicals is due to their unpaired electrons. Any atom capable of existing independently with a single electron in its outermost shell is considered a free radical. These unpaired electrons allow hydroxyl radicals to steal electrons from other molecules or compounds with corresponding unpaired electrons, resulting in the formation of new free radicals.

Activated carbon filters

Activated carbon is a carbonaceous material that has been treated with high temperatures and sometimes chemical agents to increase its surface area and porosity. This treatment process is called activation and results in the creation of a network of small pores. The porous structure of activated carbon enables it to trap pollutants through a process called adsorption, where ions or molecules stick to the outside of the carbon molecule. The increased surface area of activated carbon provides more space for pollutants to adhere to, improving its ability to trap and remove contaminants from the air.

In addition to VOCs, activated carbon filters can also reduce odours caused by cooking, pets, mould, and cigarette smoke. They can be used in conjunction with HEPA filters to provide comprehensive air purification, eliminating allergens, bacteria, and other harmful contaminants. It is important to note that activated carbon filters need to be replaced regularly, typically every six months, to maintain their effectiveness and prevent the release of trapped pollutants back into the air.

Cartridge filters

The construction of a cartridge filter typically involves several components: the filter element, the support core, the end caps, gaskets, and the housing or casing. The filter element is made of a porous material that captures impurities. The pore size and material of the filter element can be chosen to achieve the desired level of filtration. The support core provides structural support and helps maintain the filter's shape during high flow rates. The end caps seal the ends of the filter element and provide a mounting point within the housing. Gaskets create a seal between the housing and the end caps, ensuring the fluid passes through the filter element.

Clean Air Delivery Rate (CADR)

The Clean Air Delivery Rate (CADR) is a figure of merit that indicates the volume of filtered air an air-filtration system delivers per minute. The higher the CADR, the more clean air the system produces, and the more efficient the filtering process is. The CADR rating is measured in cubic feet per minute (CFM) and is calculated by multiplying the airflow rate (in CFM) by the fraction of particles (of a particular size distribution) that have been removed from the air.

The CADR rating was developed by the Association of Home Appliance Manufacturers (AHAM) and is recognised by retailers, manufacturers, standards organisations, and government bodies. The defined particle size ranges are 0.09–1.0 μm for smoke, 0.5–3 μm for dust, and 5–11 μm for pollen. Three CADR ratings are typically measured for a given device, corresponding to smoke, pollen, and dust, from smallest to largest particles, and from most to least dangerous. The higher the CADR number, the more air is filtered per minute for that particle size range.

The CADR of an air cleaner should be equal to at least two-thirds of the room's area. For example, a room with dimensions of 10 feet by 12 feet has an area of 120 square feet, so it is best to have an air cleaner with a smoke CADR of at least 80. For wildfire smoke, the smoke CADR should be equal to the size of the room in square feet.

The most common air filter, the 1-inch throwaway found in 85% of homes, has a CADR of 10. In contrast, the Trane CleanEffects® Whole Home Air Cleaner has a CADR of 1200, meaning it filters all the air in a 12′ x 10′ x 10′ sized room every minute.

Frequently asked questions