## 1 Introduction  
With the rapid development of our society, air pollution caused by human activities results, including from coal burning, industrial production, and vehicle emissions [1, 2, 3]. Particulate matter (PM) in the air pollution, defined as a suspended mixture of minute solid particles (comprising SO\({}_{2}\), NOx, CO, heavy metals, and volatile organic compounds) and liquid droplets [4, 5, 6], seriously affects our living environments and has a great impact on human health [4, 7, 8, 9, 10, 11, 12, 13, 14]. In particular, PM\({}_{2.5}\) and PM\({}_{10}\), whose particle sizes are less than or equal to 2.5 \(\upmu\)m and 10 \(\upmu\)m, can enter the human circulation system through the respiratory tract, and cause damage to the lungs, heart, bronchus and other organs, and even lead to cancer [8, 15, 16, 17, 18]. Figure 1a shows a clear and a hazy day in a famous scenic spot in Beijing, where high PM concentration and poor visibility always appear. With the improvement of people's health consciousness, air filters are becoming quite necessary and indispensable.  
Fibrous filters can be made from many materials, including cellulose, glass, polymers, ceramics, or metals, which are economical and capable of efficiently removing PM from gas streams. Many polymers, such as polyurethane (PU) [19], polyacrylamide (PAN) [20], poly(lactic acid) (PLA) [21], polyvinyl chloride (PVC), polystyrene (EPS) [22], etc., have been reported in fabricating nanofiber membranes used for air filtration. However, these highly corrosion-resistant materials cannot be degraded naturally, and usually require subsequent incineration treatment, causing secondary pollution in the ambient air [23, 24]. Shellac, as a natural purple resin, is mainly distributed in Southeast Asia, such as Yunnan Provinceof China and Thailand. Shellac is generally recognized as safe by the Food and Drug Administration (FDA), and therefore can be applied as a food additive and raw material in food coatings and film formulations, which indicates its good biodegradability [25; 26; 27; 28; 29]. In addition, the price is low, at only USD 20 for one kilogram. In this study, we explored the application of shellac nanofibers in air filtration. The purified shellac is a yellow powder (Figure 1b), and the molecular structure is shown in Figure 1c [30]. According to the literature, shellac is characterized by high brittleness. However, the polar-polar interaction between two polymers can overcome the brittleness when another polymer with high polarity was introduced (Figure S1) [31]. Shellac nanofibers also have bad light transmittance (Figure S2a). To take advantage of the shellac nanofibers' biodegradability and improve their mechanical properties and light transmittance, we introduced a small amount of polar polyvinylpyrrolidone (PVP) into the shellac solution, and successfully prepared a uniform PVP-assisted shellac nanofiber membrane (P-Shellac FME) on a metal mesh (150 mesh) using electrospinning technology. The filtration efficiencies of PM\({}_{2.5}\) and PM\({}_{10}\) for P-Shellac were 95% and 98.1%, respectively, which is 35-40% higher than that the efficiencies of a surgical mask. In a word, the P-Shellac FME is an efficient, biodegradable, translucent, breathable air filter and has potential application prospect.