Adequate ventilation is absolutely essential when you cannot avoid using combustion engines in confined spaces.
Traditionally, explosive blasting has been used for demolition or breaking up rock. Blasting in a confined space can produce high levels of carbon monoxide. 
You must use mechanical ventilation and perform air tests before workers re-enter the blast area to ensure that the carbon monoxide levels are within acceptable levels.
Sulphur Dioxide (SO2) is a very irritating and corrosive gas with a strong sulphur-like odour which can be found in pulp-and paper mills, oil refineries and metal processing.
Chlorine (CL2) Is another irritating and highly corrosive gas with a bleach-like odour used as a disinfectant in water and sewage
treatment plants and a wide variety of other industrial settings.
Ammonia (NH3) Is a fairly common chemical used as a refrigerant and in making fertilizer, synthetic fibers, plastics, and dyes.
Hundreds of other toxic materials may be encountered in factories, chemical plants, and similar Industrial settings.
The best way to obtain Information regarding the presence or absence of toxic materials is to discuss the proposed work with the client or workplace and ask for the information.