Because confined spaces are poorly ventilated, atmospheric contaminants can build up to hazardous levels very quickly. For construction in an industrial setting, the type of airborne hazard that may be encountered depends on
 products stored in the confined space
 the type of work tasks performed in the confined space
 work or processes being performed near the confined space.
The most common atmospheric contaminants in construction and industrial workplaces include hydrogen sulphide, carbon monoxide, sulphur dioxide, chlorine, and ammonia.
Hydrogen Sulphide (H2S) is a gas generated by the decomposition of garbage and sewage. H2S can be found in sewers, sewage treatment plants, refineries, and pulp mills. It is also found in many oil refineries since most crude oil in Canada has some H2S dissolved in it. H2S is very toxic. A single breath at a concentration of about 500-700 ppm (parts per million) can be instantly fatal. At very low concentrations, H2S has the characteristic odour of rotten eggs.
However, at about 100 ppm it can deaden your sense of smell and create the false impression that no hazard exists.
Carbon Monoxide (CO) is a very common toxic gas. It has no odour or taste and is clear and colourless. Carbon monoxide poisoning can be very subtle and may cause drowsiness and collapse followed by death. The major sources of CO include the internal combustion engines powering saws, scissor lifts, power trowellers, generators, and forklift trucks. Even these relatively small engines produce high levels of CO.
Heating in confined areas, particularly with propane, presents special hazards and requires special safeguards. Propane is heavier than air and can collect in low-lying areas such as trenches, basements, and shaft bottoms. Propane can also be absorbed into clothing. Workers must therefore use extreme caution.  In the event of leakage or flame-out. Direct-fired heaters release combustion emissions directly into the air where people work. Although carbon monoxide
(CO) is the main concern, carbon dioxide (CO2) and nitrogen oxides may also be a problem.

See attached (at the top under the paper clip) CO levels and Symptoms