HEPA vs ULPA: What's the Difference?

Filters are a critical component of any biosafety cabinet (BSC). Understanding how filters work, what their levels of maintenance are (and why it’s important to keep them clean), and how often your filters need to be replaced, are factors that need to be considered when purchasing a BSC because they feed into the overall cost and ROI—and ultimately play a role in deciding which type of BSC is right for you.

Financials aside, there are also different filters available for your scientific needs and some filter types are better suited for some applications over others. Here, we look at the two highest efficiency filters that can be used with a BSC, the high efficiency particulate air (HEPA) filter and ultra-low penetration (ULPA) filter, how they differ from each other, and when you should use these filters in your research.

The Structure of HEPA and ULPA Filters and How they Work

Structurally and chemically speaking, HEPA and ULPA filters are very similar. These filters are used in BSCs where an airflow (powered by a motor) carries hazardous gases and particulate matter. Both types of filter use mats of randomly arranged boron silicate fibers (fiberglass) that range in size between 0.5 microns and 2 microns.

Between the mats are gaps that allow fumes, chemicals, gases, and vapors to move through the filter to an exhaust that removes the gases from the local atmosphere. Particulate matter carried by this airflow does not pass through the filter and gets stuck in the filter, removing it from the atmosphere.

The main difference between the two filters is that ULPA uses a higher density fiberglass to trap particulate matter, with higher density ULPA filters being able to trap smaller particulate matter than HEPA filters (and a wider range of particulate matter). The trapping of particles, and different-sized particles, in either filter is not solely reliant on the size of the openings present in the filter (unlike a sieve for example).

If BSC filters only followed a size-based strategy to remove particulate matter from air, then the holes in the filter would quickly fill up, degrading the performance of the BSC, putting more strain on the motor, and requiring a more frequent filter replacement. Both filters also rely on a number of physical mechanisms, including electrostatic attraction, that cause the particles to become stuck on the fibers inside the filter. This approach means that the filters don’t become blocked often and therefore have a long life span, with HEPA filters typically having a longer life span than ULPA filters.

Removal of Particulate Matter

The function of filters is to remove particulate matter; different types of filters will remove different sizes and levels of particulate matter in the air, and there’s sometimes a trade-off in other characteristics if you want a much greater degree of particulate removal from the air. Both HEPA and ULPA filters are governed by NSF/ANSI 49 standards and the different filters must have certain characteristics and reach certain performance levels to either be classified as a HEPA or ULPA filter.

The Standard NSF/ANSI 49 specifies that C or J type HEPA filters need to be used in a BSC, whereas for ULPA filters, type F and K filters must be used. The filters need to meet testing standards set out using the IEST-RP-CC001 test method. Using these methods, type C HEPA filters must remove particles 0.3 microns in size with an efficiency of at least 99.99% and J type HEPA filters must remove 99.99% of particles from 0.1 to 0.2 microns or from 0.2 to 0.3 microns. All HEPA filters can remove particles above and below these particle size ranges, but the efficiency outside these ranges varies.

For ULPA filters, type F filters must remove 99.999% of contaminants either between 0.1 microns and 0.2 microns or between 0.2 microns and 0.3 microns, whereas a type K ULPA filter must remove 99.995% of particles from 0.1 to 0.2 microns or from 0.2 to 0.3 microns. Like HEPA filters, ULPA filters can remove particles that are smaller and larger than the test size but the testing for ULPA filters sets a minimum efficiency, so ULPA filters will always remove more particulate matter than HEPA filters from the air. Under EN1822-1, the necessary efficiency is 99.9995% at the most penetrating particle size (MPPS) to be classified as a ULPA filter.

Cost Differences

When it comes to choosing a BSC and customizing any additional features to suit your needs, one of the natural drivers toward making a decision is cost—and this is often closely governed by the budget available to each person/organization. Overall, ULPA filters are typically 35% more expensive than HEPA filters of the same dimensions.

However, there is more than just the cost of the filter itself to consider. ULPA filters typically have shorter service lives than HEPA filters—a 10-15-year average for HEPA filters compared to 5-8 for ULPA—so there’s going to be more money spent on maintenance costs and filter replacement when using ULPA filters over HEPA.

The higher density of ULPA filters means that there is more resistance generated in the airflow dynamics of the cabinet. This is typically overcome by either using larger blowers or multiple blowers, which again cost more to install and maintain compared to a single, standard-size blower. Additionally, 20-50% less air flows through a ULPA filter compared to a HEPA filter. To move the same volume of air though a ULPA filter, it takes a lot more force to do so than when a HEPA filter is used (assuming same dimensions).

Therefore, to maintain the same level of airflow and protection using a ULPA filter instead of a HEPA filter, either the filter depth needs to be increased, the filter area needs to be increased, or the power of the motor/blower needs to be increased (as mentioned above). On increasing the filter depth/area, it will increase the amount of air that can pass through, but you may need to increase the size of your BSC, which could increase costs significantly—but all three preventative measures will increase the cost of your BSC at both the point of purchase and in operational expenses.

HEPA Filters: The Go-to For Biological Research

Applications and scientific fields should always be considered with any piece of equipment, and a seemingly simple particle filter is no different. HEPA filters are the standard high-quality filter for biological research and if any research is taking place that involves particulate matter, hazardous substances, or pathogenic material, then a HEPA is critical to ensure the safety of your work zone (protecting it from contamination) and the safety of the open lab environment.

HEPA filters remove airborne particulates, allergens, bacteria, and other materials that could cause health problems, or cross-contamination, if allowed to remain airborne. HEPA filters are efficient filter mediums, but the effectiveness of any HEPA filter is ultimately determined by the surface area, or the size, of the filter and the number of air exchanges through the filter. They have now become a standard option for biological research labs, hospitals, clean rooms, and other medical facilities.

ULPA Filters: Better Suited to High-tech Manufacturing

Common logic might dictate that having a higher efficiency that can remove more particulate matter would be desirable when working with hazardous biological matter. However, this is not the case because the additional particulate removed by a ULPA filter would be either smaller, or at lower concentrations, than is necessary in biological research. Instead, ULPA filters are mainly intended for use in industries where the removal of smaller or more dispersed particulate is much more critical. One notable example is the semiconductor industry where small amounts of dust can significantly affect the fabrication of a device and subsequently its performance.

For biological applications, not much is gained by using a ULPA filter over a HEPA filter. Microorganisms and viruses tend to group together as large particles or become attached to other particles in the air so there’s no need for the smaller particle removal features of ULPA filters. As mentioned above, ULPA filters affect the flow dynamics of the cabinet and require larger motors. This can increase the noise level and vibration within the cabinet, making it less suitable for biological research and sharing workspaces.

Finally, ULPA filters also require a different testing protocol using equipment that is generally not maintained by biological safety cabinet certification companies—meaning that extra tests are needed, making it less desirable for biological applications compared to HEPA filters. These tests are typically performed using aerosolized polystyrene latex spheres of specific size. The spheres are introduced to the air stream and a laser particle counter then measures the size and number of airborne spheres to determine efficiency of the ULPA filter.

Which One Is Best?

The answer to this question is subjective and is strictly down to the individual needs of companies, researchers, and academic research labs. Whether or not the added efficiency, but added cost, can be justified can only be answered by you—as you know your own personal circumstances. If you’re involved with biological research then the HEPA filter is the obvious choice because they last for longer, are lower cost, and reduce the overheads associated with accommodating ULPA filters (including potentially larger BSC cabinets).

However, if you’re in a work environment where dust particles are going to affect the performance of a device, manufactured good, research project, or product that you’re making, then the extra cost and particle removal efficiency offered by ULPA filters will be justified and will likely be the best option for ensuring a high product quality.