Discussion
Our study shows that wearing a surgical mask may reduce the result of SST in a cohort study of young patients without any olfactory dysfunction. All the subjects, except one, had a significantly better total score (TDI) without a mask. The average standard deviation for the TDI score with and without a mask, was inferior to 5, making it non clinically relevant (6). Nevertheless, for 8/20 subjects (40%) the TDI difference with and without mask was superior to 5. Moreover, 4/20 subjects (20%) of the participants normosmic without a mask, became hyposmic with a mask.
With the Covid-19 pandemic, wearing a mask in daily life has become a worldwide recommendation (1). The potential decrease in the sense of smell by wearing a mask must be evaluated, because of its impact on the quality of daily and professional life.
The pathophysiological mechanism of this mask-induced hyposmia is unclear. Considering that the filter of a surgical mask blocks particles greater than 0.9 µm (7), while odorous particles are nanomometer sized, the hypothesis of a particle-filtering barrier cannot be retained. Another hypothesis would be the reduction of the air flow reaching the olfactory cleft. Besides the direct filtration of particles, the protective effect of the mask is also based on electrostatic deposition, occurring due to a charge difference between a fiber and a particle (8). Odorant molecules are electrically charged and could be blocked by the mask, also electrically charged.
The most impacted sub-test was the discrimination test, while threshold and identification tests were less impacted. The mask could filter some odorant particles and not others, according to their molecular size and electric charge. The identification test was the less impacted sub-test because odor identification requires not only odor perception but also a cognitive function (9).
Chen et al., in a comparative study with and without a mask, noticed a preserved odor-identification with a decreased T-score (10). Nevertheless, they did not perform a discrimination sub-test, thus risking underestimating the impact of a mask on olfaction (10).
We decided not to perform a quality-of-life questionnaire, because not applicable for a reversible acute problem. Nevertheless, imagining a chronic sensory hypostimulation, future investigations would be interesting to know if daily face mask use, over a period of years, has a definitive impact on olfaction. The impact of mask must also be evaluated to assess if the chronic use of a mask could potentially reduce or slow down the olfaction recovery in case of Covid-19 dysosmia.
Our study has some limitations. Firstly, our study is a pilot study, which explains the small number of patients. Secondly, our population was young and mostly normosmic. Future studies with older patients as well as hyposmic patients and quality of life questionnaires would be interesting.