3 Design Considerations for Manufacturing Medical Grade Face Masks
Surgical masks and respirators are an essential part of the personal protective equipment used in the healthcare sector to combat respiratory infections. They're worn over the nose and mouth to prevent the spread of respiratory bacteria and viruses. The two common types of medical masks used as PPE, surgical masks, and N95 respirators, are each classified by filtering capacity. Surgical masks are loose-fitting and protect against respiratory infections spread by droplets through coughing and sneezing. N95 respirators are tight-fitting and block 95% of the airborne particles to prevent the wearer from inhaling airborne infectious particles.
1. ASTM Testing Criteria
The American Society for Testing and Materials (ASTM) develops and sets testing criteria for all types of medical masks. These standards encompass the design, classification, performance, user instructions, and labeling requirements to guide medical mask manufacturers.
The ASTM uses four main attributes to test medical masks and ensure compliance with the set standards.
- Fluid Resistance - This test measures the mask's ability to resist penetration by a small amount of synthetic blood (about 2 ml) at different velocities. Medical masks are tested at 80, 120, and 160 mm HG velocities. Fluid resistance also evaluates a mask's ability to contain respiratory droplets from coughing and sneezing.
- Particle Filtration Efficiency (PFE) - This test measures the mask's capacity to filter out particles bigger than 1 micron. It takes place in a chamber with controlled airflow and uses latex aerosol fluids. PFE is expressed as a percentage and ranges from 95-98%.
- Bacterial Filtration Efficiency (BFE) - The test evaluates a mask's capacity to filter bacteria larger than 3 microns. It uses the Staphylococcus aureus, the pneumonia-causing microbe, and the most dangerous of all staphylococcal bacteria. The results are expressed as a percentage with the best mask filtering out 95-98% of the bacteria.
- Differential Pressure (ΔP) - This test measures a mask's capacity to resist airflow or its breathability. The test entails driving a controlled airflow through the mask and measuring the pressure difference on both sides. A lower breathing resistance translates to a higher breathability and comfort lever for users.
2. Levels of Protection
Medical masks are evaluated for design and performance in each of the four criteria before receiving a stamp of approval. ASTM classifies approved medical masks into three categories: Level 1, 2, and 3.
- ASTM Level 1 masks offer low barrier protection. They're used for exams and short procedures with a low risk of spray, aerosols, or fluids. These base medical masks filter more than 95% of particles and bacteria, resist synthetic blood penetration at 80 mm Hg, and offer exceptional breathability. Level 1 masks are easily identifiable by the two ear loops and are designed for use in isolation units, hospital floors, labor units, and more.
- ASTM Level 2 medical masks offer moderate barrier protection. They are used in procedures that produce light to moderate amounts of aerosol, fluids, and sprays. These masks filter more than 98% of the particles and bacteria, resist fluids fluid penetration at 120 mm Hg, and have good breathability. Level 2 masks are suitable for bedside procedures in intensive care units and emergency departments.
- ASTM Level 3 medical masks offer maximum barrier protection. There are designed for procedures with higher levels of fluids, aerosols, and spray. They filter out more than 98% of the particles and filters, resist penetration by liquids at 160 mm Hg, and have good breathability. Level 3 masks are primarily used by the operating room staff and are designed for use in procedures with a high risk of exposure to fluids.
3. Filtration Materials
Medical masks typically have three or four layers of synthetic nonwoven materials and are designed to nestle filtration layers in the middle. The fourth layer is usually found in Level 3 medical masks to provide the best possible protection.
Common materials used to make the layers of a medical mask include:
- Meltblown layer - Used to make the filtration system in the middle of the mask.
- Cellulose - Used to create inner and outer layers.
- Spunbond polypropylene - Used to create the inner and outer layers and to provide an extra protection layer.
- Thermalbond polypropylene - Used to make the inner layers.
The deadly coronavirus is the latest threat to confront healthcare workers, and it's driving manufacturers to innovate and upgrade the current face masks. Materials scientists, engineers, biologists, and chemists have banded together to develop the latest series of masks that can do more than filter out particles. They can sterilize air, function diagnostically, and last longer than current disposable masks.
Healthcare workers face a continuous and evolving threat from highly infectious respiratory diseases. As a result, medical mask manufacturers are continually innovating and improving their designs in response to the growing risks.