How well are you protected? Face masks and visors
29 October 2020
THE MATERIALS from which the masks are made, and, in particular where breathable air is inhaled, is of great importance when considering the type of barrier required to inhibit possible ingress of dangerous particles, liquid droplets and solid material dusts.
Face masks have been around for many years and are used in many trades and professions. The use of a face mask is to protect the wearer from inhaling or ingesting foreign airborne matter.
Masks, which are used in industry, are broadly graded according to the type of protection provided. For the types used to inhibit the ingress of general dusts they are assigned Filtering Face Piece Level 1 (FFP1). Where dust particles are of a finer texture a mask with a FFP2 could be appropriate. The top grade in this rating system is the FFP3 and often, because of the more restricted airflow for the wearer, the mask housing may incorporate a breath exhalable pressure relief valve. Due to the better air filtration properties of the FFP3 mask, it can be used in the medical field for example, in situations where treating virus infected patients.
Medical and Non-Medical Fabricated Face Masks: Designs and Construction
The nature of masks with highest levels of protection is often a firm moulded plastic outer material. Internally they may have flexible, replaceable multi-layer materials. Using a different filtration rating code, a top graded mask would be N95. This means that the mask impedes 95% of microscopic small liquid droplets that could contain viruses from being inhaled, or conversely, the possibility of only 5% of the droplets making ingress into the mask breathing space.
N95 respirators made by different companies were found to have different filtration efficiencies for the most penetrating particle size (0.1 to 0.3 micron), but all were at least 95% efficient at that size for NaCl particles. Above the most penetrating particle size the filtration efficiency increases with size; it reaches approximately 99.5% or higher at about 0.75 micron. Tests with bacteria of size and shape similar to Mycobacterium tuberculosis also showed filtration efficiencies of 99.5% or higher.
The most basic form of mask construction is a homemade mask
which uses a few layers of woven cloth (suggestions have circulated online such as a t-shirt or dress material etc.). This type of fabric has a low thread count per square inch and when used for a mask, even multi-layer fabrication has little resistance to induced breathable airflow and likewise impedance to liquid droplets, hence a notional mask N rating would be quite low. Some people might take the view “better than nothing”, but in higher risk situations, that view could be very dangerous.
There are intermediate-level constructed masks available which can meet low risk situations. These masks are inexpensive, even when purchased in small quantities, and can be single use and disposable. The usual construction of these masks uses a non-woven polypropylene (NWPP) plastic material which from an airflow point of view is relatively porous in nature.
FFP1 mask: FFP2 mask: FFP3 mask:
Factors in mask designs and usages
To maximise the best protection from a face mask, it needs to have a close fit around the contours of the face of the wearer and beneath the chin. If breathable air is allowed to enter other than through the filtered inlet of the mask, it defeats the object of wearing it. Because there is no universal face contour, the edges of the mask must be made flexible, particularly around the bridge of the nose. A flexible metal strip is included in that area and can be easily adjusted by the wearer, to provide a close fit. Additionally, by ensuring a close fit, it prevents those who wear glasses from steaming up.
In addition to using face masks, government health advisors have advocated the additional protection of also wearing face visors. This is because if a virus carrier sneezes or coughs in the vicinity of others, those who are wearing masks have a breathable air protection. However, the droplets containing virus may be projected onto skin surfaces, in particular around the eyes. It may seem a good solution to wear goggles but these are an encumbrance for various reasons.
Transparent visors do offer protection for this problem. A wide range of visors is available with different construction approaches, ease of use and durability. Visors made with polycarbonate of a reasonable thickness are robust and easy to wipe clean. Other factor when choosing a visor is the quality of padding inside the headband, which rests against the wearer’s forehead.
Examples of face visors
Some other types of visors are available which combine both face mask and face visor in one unit. Generally, they are made of a more overall rigid construction. A disadvantage is that complete cleaning of internal mask components can be more difficult.
The highest level of protection has been used in the manufacturing industry for many years. This requires complete head and body covering, for example in situations where the wearer is exposed to a dangerous chemical or microbiological environment.
Breathable air supply is provided for these suits powered from external regulated positive sources. After emerging from contaminated areas, it may be necessary for the wearer to be washed down with water or neutralising chemical solutions.
It is worth mentioning that on Friday, July 24, 2020 marked the first day where face masks are mandatory in shops and supermarkets in the UK. The government announced this latest measure, ruling that wearing a face mask would be a legal requirement for shoppers in certain stores and supermarkets. There are some exemptions to the rule, including those with medical or physical conditions, which prevent them from doing so. How important to make sure that the right face mask is selected and used for effective protection.
S. Awolesi, MSc, Senior Associate Member of the Royal Society of Medicine, Environmental Science Group Limited, Cambridge. Roy Simmons, BSc, Senior Consultant, Environmental Science Group Limited, March, Cambridge.