This study was conducted to assess changes in spirometric parameters after protective interventions among workers at a chlorine production plant in Semnan, Iran during 2012-2016. METHODS: This quasi-experimental study included 100 workers at a chlorine production plant in Semnan during 2012-2016. Spirometric parameters (forced vital capacity [FVC], forced expiratory volume in 1 second [FEV1], FEV1/FVC, peak expiratory flow [PEF], and PEF occurring in the middle 50% of the patient's exhaled volume [PEF 25-75%]) were measured in all workers before the initial intervention in 2012. Protective interventions were then implemented for 4 consecutive years and the parameters were measured annually. A multivariable linear regression model was used to assess the factors affecting spirometric parameters before and after the protective interventions in SPSS version 24. RESULTS: The mean values of all spirometric parameters significantly increased after the protective interventions (p<0.05). Multivariable linear regression showed that age (β=-0.40), body mass index (BMI) (β=0.71; 95% confidence interval [CI], 0.11 to 1.31), and type of mask (β=-7.88; 95% CI,-15.96 to-0.46) had significant effects on the mean difference in FVC. Similarly, age (β=-0.35; 95% CI,-0.70 to-0.01), BMI (β=0.80; 95% CI, 0.20 to 1.41) and type of mask (β=-8.88; 95% CI,-16.98 to-0.79) had significant associations with the mean difference in FEV1. The type of mask (β=-12.81; 95% CI,-25.01 to-0.60) had a significant effect on the mean difference in PEF. CONCLUSIONS: All spirometric parameters significantly increased in workers after protective interventions were implemented. Therefore, protective interventions to prevent respiratory disorders in workers exposed to chlorine gas are suggested.