Polluted air is a major hazard in urban environments and responsible for many illnesses and deaths each year. The indoor environment also adds to air pollution in numerous ways, causing a range of complaints termed Sick Building Syndrome. Now the COVID-19 pandemic has added to the risks of working, studying or spending time indoors in a space shared with many other people.
The risk of airborne infection spread by respiratory aerosols means that social distancing and surface hygiene are not sufficient protection indoors, especially when sitting or standing in one place for long periods when other people are nearby. It’s essential to also focus on air hygiene to reduce the risk of infections.
One of the easiest and most effective ways to improve air quality is by placing mobile air purifiers in a room. Air purifiers clean the air by removing airborne particles and droplets, which may contain microorganisms. They do this by passing air through a series of filters that trap the contaminants and return the cleaned air to the room.
There are many considerations in choosing one suitable for your situation. Here, we explain some of the important terms and factors used to describe air filtration devices to help you make the right choice.
1. What is a HEPA filter?
HEPA filter stands for high efficiency particulate air filter. The term was first invented in the 1950s and later became a generic term for high efficiency filters. A HEPA filter is now a standard part of most air filters, but did you know that there are many types of HEPA and no strict control over how the term is used or how well they filter impurities from the air?
Various standards were developed for different industrial requirements and the US and Europe used different approaches to classifying them. In 2011, the International Standards Organisation created a new standard for testing HEPA filters (ISO 29463) in an attempt to harmonise all the various standards used worldwide, including the most widely accepted international standard EN1822, but it still accommodates some differences.
There is still no legal requirement; however, to apply any of the HEPA standards for filters in consumer products or control over what term can be used to describe a HEPA filter. Terms such as “True HEPA”, “Sealed HEPA”, “HEPA-like” have no fixed definition, so the consumer has no guarantee of performance when these terms are used by manufacturers to describe a product.
2. What does a HEPA filter do?
HEPA filters are designed to filter airborne particles and droplets, which consist of mineral dust, industrial and vehicle pollutants, pollen, fungal spores and bioaerosols (aerosol droplets containing microorganisms and material from living organisms). A HEPA filter doesn’t remove gaseous pollutants, so an additional filter containing activated carbon has to be added to an air filter device. This can be referred to as a VOC filter (VOC = volatile organic compound) and can be combined with the HEPA in a single compound filter, or fitted separately.
Bacteria and viruses don’t float in the air on their own; they’re carried on dust particles or in bioaerosol droplets or droplet nuclei (dried droplets) that have been made airborne by various means.
This means that a filter doesn’t actually need to be able to capture particles of the sizes of bacteria or viruses to remove them. Viruses range in size from about 0.03–0.450µm (1µm = 1000th of a millimetre) and bacteria 0.3µm–0.7mm – the largest bacteria known is highly unlikely to ever become airborne as it’s only known to exist in the stomach of a surgeonfish!
A filter, however, doesn’t work like a sieve, which is an array of holes that only lets through particles smaller than the holes. The way a HEPA filter removes particulate matter is more complex.
3. How does a HEPA filter capture particles from the air?
- Inertial impaction: the largest and high-density particles follow the air flow until the air passes around a fibre. Their inertia then causes them to follow a straight line and they impact the fibre and stick to it.
- Interception: medium-sized particles also follow the path of the air flow, but when they travel between fibres and close enough to one, they’re captured.
- Diffusion: the smallest particles follow a random, irregular path as they are small enough to bounce of gas molecules in the air. This slows their passage through the filter and increases the chance that they collide with a fibre in the multiple layers of the filter mat and stick to it.
- Electrostatic attraction: this occurs mainly with synthetic fibres, which can hold a charge. As a particle passes near a fibre with opposite electrostatic charge, it’s attracted to the fibre and sticks to it.
Figure 1. The four primary filter collection mechanisms: diffusion, interception, inertial impaction, and electrostatic attraction (source: Wikipedia)
HEPA filter ratings
There are seven grades in the HEPA filter standard: from 10–16. The highest numbers (14–16) are for industries that require exceptionally pure air for manufacturing, such as electronic parts and medical products. For domestic purposes and offices, HEPA 13 is the most appropriate, filtering 99.95% of particulates from the air (for the European standard and ISO 35H).
HEPA filters are tested for conforming to the standard using 0.3µm particles because that’s the size that’s most likely to pass through a filter – called the most penetrating particle size (MPPS). So both larger and smaller particles are filtered more efficiently than the 99.95% filtration rate required to meet the HEPA 13 standard.
When a filter is in use it starts to capture particles on the fibres, which start to fill the spaces between the fibres and can make the filter more efficient. This action also lowers the airflow through the filter. Eventually, however, the filter will need replacing to dispose of the trapped dirt — including pathogens — to prevent it releasing dirt and microbes back into the air and to improve the air flow. This is why air purifiers need periodic servicing.
4. What does CADR mean?
CADR stands for clean air delivery rate and is a measure of the volume of purified air that passes through an air filter in a set time, usually measured in cubic metres per hour or cubic feet per minute (CFM) in the US. The higher the number, the faster an air filtering device can clean the air in a room.
An increase in filtering efficiency reduces the airflow through the filter, which in turn increases the energy required for a fan to blow air through. In addition, when the speed of air flow is low the airborne particles are more likely to adhere to the fibres and not bounce off them due to their kinetic energy or become dislodged by the moving air. So there’s a play-off between the volume of air processed, the filtration efficiency and the energy used.
Of course, there’s a balance between the CADR and the space or size of room that’s appropriate for that device. A portable air filter can be used to clean the air in a whole room or in hotspots, such as near a desk or seating area. The following factors need to be considered for siting an air filtration device and choosing the number required.
- Is the requirement to clean the air in a whole room or local spaces?
- What’s the volume of the space that requires purified air?
- What’s the natural air flow and ventilation in the room?
5. What is indoor air quality?
Indoor air quality or IAQ is a general term to describe the total effect of pollutants that can contaminate the air inside buildings, including both gaseous and particulate. It’s affected by building materials, furnishings and equipment used inside the building, the personal products of people occupying the building and pollution brought inside through ventilation with outside air.
These can add a wide range of substances to the air people breathe, including volatile organic compounds such as formaldehyde, industrial solvents, perfumes and biological contaminants including bacteria, viruses, fungal spores and allergenic matter such as dust mites and pollen.
Indoor air quality also relates to the effects of the pollutants on the health and wellbeing of the people working or living inside a building and how to mitigate those effects. Health effects range from headaches and dizziness to long-term illnesses such as heart disease and cancer.
Read more about IAQ in our blog on the impact of air quality on health and wellbeing.
A portable air filter can give a continuous indication of IAQ by incorporating sensors that measure factors such as VOC content, PM2.5 (particulates 2.5µm diameter) and humidity and displaying the values.
Find out more about the factors affecting air quality and choosing a suitable air purifier in our whitepaper on air hygiene.