Air filters operate in versatile environments, such as different relative humidities (RH), temperatures. Those factors could affect the state of atmospheric hygroscopic salt pollutants, mainly ammonium sulfate and ammonium nitrate. Air filters operation performance can be affected by operation environments and the state of atmospheric pollutants. However, current filter testing standards overlook the variation of filter operation environment and the diversity of atmospheric pollutants. This study aims to research the RH effect and hygroscopic salt particles effect of nanofiber coated air filter operation performance. A nanofiber coated cellulose filter media was tested with potassium chloride, ammonium sulfate, and ammonium nitrate particles in both dry and wet states. Testing RH covers from below salts’ efflorescence RH to above deliquesce RH to achieve different particle states. Both filter loading curves and the volume loading at 4 in. of water were compared between different testing conditions. The results are also compared with conventional cellulose filter media results. Generally, the higher the testing RH, the greater the volume loading, and this applies to both nanofiber coated cellulose filter media and conventional cellulose filter media regardless of salt particle states. The volume loading of dry particles is greater on nanofiber coated cellulose air filter than conventional cellulose filter, whereas the volume loading of wet particles is greater on conventional cellulose filter than nanofiber coated cellulose filter. This is because of interactions between particle states, moisture activities, and filter structures. This study reveals that the filter selection should be based on operation environments and the pollutant state to achieve a longer service life of air filters. A moderate increase of the operation RH could prolong air filters’ service life.