The researchers tested a wide range of mask products and found that nearly all would increase the daily intake of microplastic fibres during wear because of their relatively fragile structure.
The fibres could cause some health problems, but this possibility was dwarfed by their benefits during the pandemic and should not prompt people to stop wearing them, according to the researchers.
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“It is a minor problem compared with protecting humans from Covid-19,” said the team from the Institute of Hydrobiology in Wuhan in a peer-reviewed paper published in the Journal of Hazardous Materials on Wednesday.
Scientists first discovered microplastics in the lung tissue of some patients who died of lung cancer in the 1990s, and many other studies have since highlighted the potential damage to health caused by such materials.
Plastic degrades slowly, so once in the lungs it tends to stay there and build up in volume. Some studies have found that the immune system can attack these foreign objects, causing prolonged inflammation that can lead to diseases such as cancer.
Face masks help prevent airborne transmission of the coronavirus using a combination of fabrics. The commonly used surgical masks, for instance, have three layers of melt-blown textiles made of polypropylene.
Some people also use home-made cotton masks or fashion masks made of organic polymer, which offer less protection but greater comfort for everyday use.
The Institute of Hydrobiology researchers in Wuhan counted the plastic fibres shed by masks over a month, using a laboratory device that simulated human breathing. They found that masks containing activated carbon produced the highest number of fibres, at nearly 4,000.
Those were followed by surgical masks, cotton masks, fashion masks and N95 masks, the brands of which were not specified in the study. Some of the fibres were several millimetres long, considerably larger than typical particles floating in the air.
There were already plenty of microplastics in human living environments. About a third of the floating particles in the atmosphere are plastic, according to some studies. They could come from almost anywhere, from the clothes people wear to farms using plastic membrane.
Masks can filter air pollutants to varying extents, but with the exception of N95s, all produce more microplastic fibres than they filter, according to the new Chinese study.
The N95 respirator studied released only about half of the quantity of microplastics of a surgical mask. N95s are designed to filter at least 95 per cent of airborne particles. Intended mainly for medical or industrial use, they are made to higher standards with a stronger structure.
But wearing an N95 for a long time could cause fatigue because of oxygen shortage, it has been warned, and in many countries these masks were reserved for use by frontline health workers.
The institute’s study also found that reused masks produced more loosened fibres. The worst structural damage was observed after applying alcohol.
Cleaning masks by exposing them to ultraviolet light caused the smallest amount of loosening, while washing a mask gently without soap could cause some damage but not much, the researchers found.
The exact health side-effects caused by broken-off fibres is unclear, so although people should be informed of the inhalation risk they should continue to wear masks, the researchers said.
Used masks have also added to litter and recent studies have warned of their contribution to water pollution, potentially adding to consumption of harmful plastics by humans and animals.
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