Very low humidity can create discomfort, respiratory problems, and aggravate allergies in some individuals. Low humidity causes tissue lining nasal passages to dry, crack and become more susceptible to penetration of rhinovirus cold viruses.[37] Extremely low (below 20 %) relative humidities may also cause eye irritation.[38][39] The use of a humidifier in homes, especially bedrooms, can help with these symptoms.[40] Indoor relative humidities should be kept above 30% to reduce the likelihood of the occupant's nasal passages drying out, especially in winter.[38][41][42]