In the context of COVID-19, wearing a mask is essential for outbreak prevention and control. Healthcare workers are required to wear masks for prolonged periods. Previous studies have shown that prolonged mask-wearing can have various adverse effects, including headaches and skin damage [8, 9]. Still, there are fewer reports on whether mask-wearing affects olfactory.
This study shows that wearing a mask reduces odor sensitivity but does not significantly affect odor recognition. Also, there was no significant difference in the impact of the PAPRs compared to the N95 mask on the olfactory threshold test. Both masks increased the olfactory threshold. The reasons for this result were not exactly the same between the two masks, which need to be explored further.
Of all the human sensory systems, the sense of smell is the most difficult to understand. The human olfactory system consists of millions of olfactory neurons arranged in sensory epithelial cells located in the nasal cavity and is closely related to neurological and psychological aspects. A variety of reasons can cause olfactory impairment, upper respiratory tract infections, head trauma, and nasal-sinus disease being the most common causes. Recent studies have found that reduced olfactory function is also associated with a range of neurodegenerative diseases, such as Alzheimer's disease. It has been established that breathing is an integral part of the olfactory system. It plays a crucial role in detecting and perceiving odors in the olfactory system [11, 12]. Nwosu et al. noted that the discomfort of the mask on breathing might be due to the increased resistance to breathing, increased temperature, humidity, and carbon dioxide in the dead space of the mask, or the pressure of the straps. Cardiopulmonary exercise capacity and lung function parameters are significantly reduced in healthy volunteers, whether wearing surgical masks or N95 masks. Therefore, we speculate that the effect of masks on reduced olfactory function may be related to restricted respiratory physiology.
Some scholars refer to the physical and psychological deterioration and multiple symptoms after wearing masks as Mask-Induced Exhaustion Syndrome (MIES). It is accompanied by typical changes and symptoms, such as an increase in breathing dead space volume, increase in breathing resistance, increase in blood carbon dioxide, decrease in blood oxygen saturation, etc. . These changes in respiratory physiology have a significant impact on olfactory function. Hypoxia, excessive CO2, and low nasal airflow may have an effect on the sense of smell. It has been demonstrated that chronic intermittent hypoxia induces olfactory impairment while altering the activity of the main olfactory bulb neural network and its response to odor. Huppertz et al. Found a decrease in olfactory sensitivity under normobaric hypoxia. In contrast, the ability to discriminate odors was not impaired, which is very similar to the results of our study. He suggested that hypoxia can be considered a stressful situation for the human body, which leads to an increased activity of the hypothalamic-pituitary-adrenocortical axis (HPA). The N95 mask consists of four layers. A significant decrease in PaO2 and an increase in respiratory rate occurs after 4 hours of wearing the N95 mask, which may be related to the shortness of fresh air for inhalation. The combination of low oxygen and high CO2 contributed to the effect of the N95 mask on the sense of smell.
The results of Salati et al. demonstrated that an N95 mask caused excessive CO2 inhalation by approximately 7x greater per breath compared with normal breathing. Several studies on marine organisms have found that high CO2 seawater reduces the olfactory sensitivity of fish and macroinvertebrates. It is related to the interference of high CO2 with brain neurotransmitter function and the reduced affinity of odorants for their receptors at high CO2[19, 20]. Some studies showed that End-tidal CO2 increased significantly after wearing an N95 mask alone, but it was significantly mitigated when used in combination with a PAPR and the rise in CO2 was consistently lower when wearing the PAPR compared to the N95[21, 22]. This may be due to the positive pressure generated by PAPR through increased oxygen concentration in the hood and positive pressure assisted expiration. But the mechanism of how hypercapnia affects the olfactory function in humans remains to be investigated.
In addition, when wearing an N95 mask, heat exchange between the mucosal wall and the inhaled air were reduced, influencing the perception of nasal patency. The main mechanism that produces the sensation of nasal patency is a thermal receptor activated by cooling the nasal mucosa. Both ambient air temperature and humidity significantly modulate an individual's perception of patency through heat loss in the nasal mucosa and trigeminal sensory input[23, 24]. While wearing a mask, the humidity and temperature inside the mask increase compared to the air, which impacts the nasal patency, further affecting the perception of a smell. Airflow also plays a role in the human olfactory. Yao et al. indicated that nasal flow spontaneously engages central olfactory processing and is an integral part of the olfactory percept in humans. Fu et al. reported that alteration of nasal airflow affected odor thresholds distinctly. Oka et al. found that the nasal airflow rate significantly affected the responses of glomeruli in the mouse olfactory bulb. Lee et al. demonstrated an average increment in inspiratory and expiratory airflow resistance of 126% and 122%, respectively, and an average reduction in air exchange of 37% when using the N95 mask. This is objective evidence that wearing a mask can cause substantial damage to nasal airflow. PAPRs have a greater advantage in mitigating increased temperatures and humidity, which is associated with the respirator circulating fresh air into the mask. However, at the same time, 9 (32%) subjects experienced nasal congestion and dryness after wearing the PAPR compared to 2 (7%) in the N95 group(P༜0.05). This may be the main reason for the olfactory changes caused by the PAPR. Most of the existing studies have focused on PAPRs induced eye dryness[29, 30], and we suspected that adding a humidification device to the PAPR may improve these symptoms.
Furthermore, the sense of smell is adaptive. Same as vision, when our eyes are exposed to very intense light, our visual threshold rises considerably, and it takes tens of minutes to return to entirely normal. This phenomenon, known as “dark adaptation," has been studied for decades. Olfactory adaptation leads to a temporary inability to perceive a specific odor after prolonged exposure to the sense of smell. It reduces the sensitivity to perceive odors and raises the detection threshold. Therefore, we guess there is also an olfactory adaptation process before and after wearing the mask. It may take some time for people to regain their original threshold of olfactory recognition after removing the mask, but how long this time is and how different types of masks affect the length of time is not yet known.