Background: The Korea Centers for Disease Control and Prevention (KCDC) has identified cases of people suspected of suffering lung disease potentially caused by chloromethylisothiazolinone/methylisothiazolinone (CMIT/MIT) used in humidifier disinfectants (HDs). The Korean Ministry of Environment (MoE) epidemiological investigation and toxicity test study found that HDs caused health damage such as asthma and lung disease. Objectives: The main purposes of this study were to classify the HD exposure rating and to analyze the exposure characteristics that affect exposure to CMIT/MIT HDs. Methods: The exposure characteristics and socio-demographic characteristics of victim participants using CMIT/MIT HDs were investigated through questionnaires. An inhalation no observed adverse effect level (NOAEL) for CMIT/MIT was produced based on inhalation toxicity values. Exposure ratings (class 1~class 2) were cross-tabulated with clinical ratings (acceptable~unacceptable). A correlation analysis was conducted with the main exposure characteristics that affect the exposure concentration of CMIT/MIT HDs. Results: The concentration in indoor air of CMIT/MIT was 8.75±25.40 μg/m3, and the exposure concentration was 2.30±6.29 μg/m3. The CMIT/MIT exposure rating of 67 participants with high exposures of not more than MOE 100 were evaluated as 14.5%, while the damage participants who matched the clinical rating made up 4.5%. The exposure concentration of CMIT/MIT showed a positive correlation with the daily usage amount and usage frequency, and a negative correlation with volume of the indoor environment. Conclusions: A new exposure rating could be suggested and calculated based on the MOE, and the factors affecting the exposure concentration could be identified.