Good article from Safety Smart on the subject.
Protect Yourself From Respiratory Hazards » SafetySmart
Many times, persons actually doing the job also ignore the advice and try to do the job saying that it is a 2 minute job. And, they would never see the light again. Before carrying out any job, particularly in closed spaces, one should measure the concentration of all possible pollutants and also oxygen in the work atmosphere, particularly at the actual place of the work. Pollutants should be exhausted by proper ventilation so that oxygen levels are within permissible range. If it is suitable, one should enter the space by following all precautions. Else, they will become victims of dangerous work place. Though, people say 19.5% of oxygen is sufficient to enter a confined space, but still it is below 21% level in atmosphere. We should find out which are the pollutants in the work place that caused reduction in oxygen level to 19.5%. We know that carbon monoxide at 5000 ppm (0.5%) is fatal.
It is better to ensure oxygen levels nearer to atmospheric levels of 21%, else one should use self contained breathing apparatus (SCBA) so that one can work safely.
Apart from confined space jobs, respiratory protection is also required in jobs where fumes, mists, dust, etc are likely to be generated / present in the work place. Engineering measures should be taken and then as a last line of protection, one can go for personal protective equipment, in this case, use of respiratory protection.
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