The answer, in short, is it will vary depending on the environment. Heavy usage or dirtier environments require HEPA filters to be changed more frequently than light usage in clean environments.
HEPA stands for high efficiency particulate air. According to the EPA, this type of air filter can theoretically remove at least 99.97% of dust, pollen, mold, bacteria. It can also remove any airborne particles with a size of 0.3 microns or larger. Using data we can speculate on how long these filter will last in different situations.
Understanding Pressure Drop, Filter Loading and Motor Design
Let talk pressure drop across the filter. As you can imagine, pushing air through a tightly woven filter requires a fan motor to work hard. As filters get dirty, or you add prefilters, the fan motor needs to increase the pressure to keep the air moving through the filter. The more clogged a filter gets (known as filter loading) the harder the fan has to work to move the same quantity of air. The pressure then drops as it get to the other side of the filter and is unobstructed.
Fan motors are required by NSF/ANSI 49 to be able to handle a 50% decrease in pressure across the filter. They must also maintain a less than 10% decrease in airflow. Some systems will utilize a larger motor that is able to overcome larger pressure differentials to increase the lifespan of the filters. It can do this by continuing to move the required amount of air even when the filter becomes more resistant due to particle buildup. The downside to this strategy is it can use as much as 2-3 times as much energy, which can create a costly electrical bill.
So Just How Long Does A HEPA Filter Last?
Well, it really depends on 3 factors.
1. Manufacturer Recommended Life Expectancy.
2. Amount Of Air Being Filtered: 24/7 operation or normal working hours operations.
3. The Hoods Environment: Is it in a classroom setting, clean room setting or a quality control manufacturing setting.
Note: Because of these variables it is always best to have your HEPA filter inspected by a professional.
On average a HEPA filter will last anywhere from 3 to 10 years but on average will be around 5-7 years. Nuair performed a study that took into account blower motor performance and measure how long a HEPA filter would last. This study was done on biosafety cabinets. The results are below where % equals percentage increase in pressure drop:
50% = 3 Year Filter Life
100% = 5 Year Filter Life
180% = 7 Year Filter Life
250% = 10 Year Filter Life
Replacing HEPA Filters
You should check your High-Efficiency Particulate Air (HEPA) filters every month. During your monthly check, make sure to look for punctures and potential airflow issues. It is becoming commonplace for units to have an alarm that triggers when airflow is low. However, these alarms should not be solely relied on. Keep thorough notes of replacements, repairs and testing in your Monitoring and Maintenance Record Forms.
Keep an eye out for separation in the filter media and the framing or any other punctures in the membrane. According to the US Department of Energy: "Two qualified methods for sealing the filter to its filter frame are elastomer gaskets and gelatinous seals. Two different methods or materials shall not be used on the same filter case."
Prior to removal, the filter must be decontaminated of hazardous microorganisms through the use of formaldehyde gas. For the health of users and life of the equipment, it is important to replace the filter in accordance with the filter’s predictive life.