02 November 2021
David Moulton, UK managing director of Camfil, looks at the metrics and considerations that aim to help combat dirty air.
‘Hands, face, space, ventilate, filtrate’, these are the most typical messages we hear when we are advised how to protect ourselves from hard-hitting viruses such as COVID-19. While we all understand how to wash our hands, the importance of wearing face masks and social distancing, the question remains, do we really understand the significance of good ventilation and what guidelines should we follow when implementing it?
To put it simply, ventilation is the movement of external air throughout a building. Filtration on the other hand, is the movement of purified air. Good ventilation is key in terms of protecting people from both external and internal contamination risks, so let’s first examine the difference between an external and an internal contaminant.
External contaminants are generated outside and these pollutants would contain both particulate matter (pollen and dusts) as well as gaseous contaminants (nitrogen dioxide and sulphur dioxide). The levels of these outside contaminants are dependent on the location of a building.
Internal contaminants are where the source of contamination lies inside. These contaminants can be man-made (such as viruses) or can be gaseous contaminants such as VOCs. Right now, there is a focus on the spread of viruses, however, it is important we remain vigilant of other contaminants that put us at risk.
Once the classification of contaminant risk has been established, we can begin looking at how filtration systems can be installed. In most commercial and industrial buildings, they will utilise mechanical ventilation systems. These systems enable the user to control the amount of air that is supplied and ensure that it achieves the recommended air volume or air changes required to be classified as a well ventilated building. These systems can also control the temperature of the air and help maintain the comfort levels. Other buildings may use natural ventilation, through open windows and doors to create an airflow. However, as far as guidance goes, there can be negatives associated with this as it is harder to control external pollutants from entering, making it harder to manage the airflow and air changes.
When assessing different guidelines and recommendations, there are two key metrics that are used to identify whether there is sufficient ventilation. One is the amount of air volume that is supplied. In different guidelines, we see recommendations that 10 litres per second of air per person should be introduced into an environment to ensure it is adequately ventilated. The other metric that is used is air change rate. This is in reference to how often clean air is supplied to a room or building and will calculate requirements based on the room size and not the occupancy levels. In controlled environments such as hospitals, there are often requirements for air changes to range from 5 to 20 changes per hour depending on cleanliness needs. Looking at offices and other workspaces, there are recommendations that the air changes should be between 5 to 10. The metric that achieves the higher air quality should also be utilised.
The next important thing to understand is how to know if the air is now safe, free from both external and internal contaminants. And this is where filtration comes into play. Air filtration has a major role not only in protecting from external contamination, but keeping occupants safe from internal contamination threats, such as viruses. In essence, there are three areas in a building's ventilation system where air filtration becomes important; supply air systems, recirculation air systems and in-room air purification systems.
Air filtration requirements for supply air systems
There are a number of elements that need to be considered when considering what HVAC system is best for a building. Firstly, you need to consider the condition of the outside air, how are the pollution levels? How is the location rated by World Health Organisation guidelines and recommendation levels? Secondly, what is the primary function of your building? Is there a permanent occupation of people (offices, care homes, etc.). Once these elements are understood it is easier to choose filtration levels. Air filtration efficiencies are determined by ISO 16890 standard. This helps to ensure that the air filters chosen meet requirements. In the majority of buildings in the UK and around Europe, a recommendation of an ePM1 60%+ would be a basic requirement. If there are other concerns such as odours or gaseous contaminants from the outside then a requirement for molecular filtration systems would be advised to ensure adequate protection for residents. Another consideration that is important in these systems is energy consumption. To manage the energy consumption of your air filters there is a Eurovent standard 4/21 that rates your air filter from A+ to E classification. These classifications are important to consider, especially when the requirement for Net Carbon Zero continues to grow.
Air filtration requirements for recirculation air systems
Over the course of COVID-19 we have seen many regulations and guidelines issued, expressing that recirculation air systems should be turned off, with the exception being only if these units are utilising HEPA filtration. The reason for this is that recirculating air systems, that do not use HEPA air filters, can often spread viruses from one location to other areas or redistribute contaminated air back into the room causing further outbreaks. The problem here is that not all systems are made to accommodate HEPA air filters. Often the increased pressure drop will either not be permitted by the fan or a reduction in air supply will occur. As a result, it is important to ensure your system is properly protected. For instance, if your recirculation air mixes through your supply air system you could be safely operate it with installing an ePM1 60% bag filter in the exhaust air, then with two further stages in the supply air system of ePM1 60% your final filtration performance will be somewhere in the region of 95%. This can often be a simple upgrade producing maximum benefit.
Air filtration requirements for air purification systems
If we examine WHO recommendations and government guidelines we see that one technology is often mentioned, HEPA filtration. HEPA filtration has been used for years to protect people from airborne contamination risks. But what exactly is a HEPA filter? I think it’s best described as a tested and certified filter that performs at 99.95%(H13) or 99.995%(H14) efficiency. When choosing a HEPA air purifier there are a number of important considerations, such as:
- Is the HEPA filter individually tested and certified to EN1822:2019 standard?
- Is the unit sized adequately for the space? REHVA recommends that air purifiers are sized to achieve between 2 and 5 air changes in your environment.
- Does the system create any bi-products? Some air purifiers that use other technology can create ozone. Check that 0% emissions are created by your system.
These are just some of the key metrics and considerations to bear in mind when assessing the viability of air filtration systems for any building in a post-pandemic world.