HEPA filters are used by mushroom cultivators to create laminar flow hoods. Their fine filtration ability is utilised to create sterile environments to prevent contamination of cultures. Contaminants include dust, dirt, fungi, bacteria, and viruses that can travel in the air. Mushrooms are highly vulnerable to infection when they are first growing so cultivation requires a clean and sterile environment.
HEPA filters have multiple layers to provide a remarkably high level of filtration. An outer layer acts as a barrier against large particles, those that can be seen with the naked eye, including dust, hair and dirt. Inside the large filter is a collection of thin filters that act to prevent smaller particles from passing through. This thinner filter is made up of minute glass fibres that can catch the microscopic contaminants from the air. The combination of the larger and finer filter layers enable the HEPA to filter against both large and small particle sizes, thus creating a sterile environment.
HEPA filters are placed at the front of the hood to filter the incoming air to greatly reduce the chance of incoming airborne contaminants and therefore to create a sterile environment. A fan is placed within another wall of the hood to draw air in and across the HEPA filter. A pre-filter, with less air resistance than the HEPA (e.g. Promedia T15), is attached over the fan on the outside of the hood. A net/covering should be attached over the HEPA on the outside of the hood to protect the delicate filter, to lengthen efficiency.
The HEPA filters recommended for laminar flow hoods are of H13 and H14 efficiency. H13 prevent >99.95% of contaminants from flowing through and have a pressure drop of 90 Pa. H14 are slightly more efficient, preventing >99.995% of contaminants and have a pressure drop of 100 Pa.