Portable humidifiers are extensively employed to increase indoor humidity; however, the generated particles could affect people indoors and cause a heavy disease burden. This study aims to compare the potential for an ultrasonic humidifier and an evaporative humidifier to release suspended materials contained in the charging water in a room-sized chamber and estimate the disease burden attributed to PM 2.5 released by humidifiers. Size distributions and concentrations of indoor particles were measured when humidifiers were filled with five types of water with varying total dissolved solids (TDS). Results showed a linear association (R 2 =0.980) between particles produced by the ultrasonic humidifier and the TDS in water. Evaporative humidifiers could produce a small number of particles when tap water was used, but the linear relationship between the released particles and TDS in water was weak (R 2 =0.028). The disability-adjusted life years (DALY) attributed to PM 2.5 generated by the ultrasonic humidifier was 22.4 times that of the evaporative humidifier when using tap water. This study provides valuable data on characteristics of particles released by two different humidifiers and highlights the potential disease burden of exposure to PM 2.5 generated by humidifiers, which may help exclude any adverse effects of using portable humidifiers. • The impact of portable humidifiers on airborne particles is explored. • Ultrasonic humidifiers can release dissolved solids from the charging water. • Evaporative humidifiers have few influences on airborne particles. • Ultrasonic humidifiers can cause heavy burden of disease when using tap water.