This paper reports a study of air environment at industrial premises where welding processes take place, with special attention paid to the formation of carbon monoxide (oxide) (CO) in the working environment in the process of manual arc welding. We have given the classification of basic harmful substances generated during welding and related processes in terms of the character of negative influence on the body of a welder. A mathematical model of the dynamics of change in the concentration of carbon monoxide in the air of a working area has been constructed, based on the amount of a harmful substance (m) in the air at premises at a time point, the intensity of its release into air, and the air exchange rate. A given mathematical model includes the propagation of carbon monoxide in the air, considering the air exchange between the overall volume of a premise and the local volumes of working zones. There are not enough studies into the formation of carbon monoxide during welding processes, which is why examining this process is a priority. Out experimental study has confirmed that the concentration of carbon monoxide outside the local volumes of local ventilation devices, that is in the air of working zones, remains constant (to 0.01 mg/m3) and does not exceed MPC (20 mg/m3). A failure or the absence of general ventilation leads to a rapid increase in the concentration of carbon monoxide (CO) in line with an exponential dependence (from 150 to 200 mg/m3 over 0.5‒0.6 hours) within a small closed workspace (1 m3), and can quickly spread throughout the entire premise. However, a failure or the absence of general ventilation leads to a rapid increase in the concentration of carbon monoxide (CO) in line with an exponential dependence. This indicates that general ventilation is important, but it does not warrant safety for welders and other workers from gas poisoning. Therefore, the use of local ventilation must be ensured, as well as respiratory protection for all present when conducting welding processes. The derived mathematical models make it possible to assess risks during welders’ operations, to take into consideration CO emissions when calculating ventilation systems in working areas, to adjust the system that manages risks and labor safety.