If you have to work in a closed environment, you must make sure that the air is safe to inhale.
 
• Is there enough oxygen in the air?
• Are there occurrences of toxic or explosion hazardous substances?

To ensure your surroundings, you should always check the environment with a gas detector. This is a precondition that your respiration equipment and gas detection instrument work in every situation. This can only happen if it is kept in repair and is controlled as directed.

We have got the answer with robust and simple solution from Biosystems by Sperian. 

            Name and meaning of chemical formulas:

Formula             Name                                                       CAS-number.

CL2                     Chlorine                                                    7782-50-5 

CLO2                   Chlorine dioxide                                       10049-04-4

CO                       Carbon monoxide                                     630-08-0 

CO-H                   Carbon monoxide at H2S occurrence       630-08-0

H2S                     Hydrogen sulphide                                    04-06-7783

HCN                     Hydrogen cyanide                                     74-90-8

NH3                     Ammonia                                                   7664-41-7

NO2                     Nitrous gasses                                          10102-44-0

O2                       Oxygen                                                      7782-44-7

PH3                     Phosphorus                                               7803-51-2

SO2                     Sulphurous dioxide                                   05-09-7446

CO2                     Carbon dioxide                                          124-38-9

LEL-                    Explosive gasses and vapours                  Lower Exposure Limit