This is the most effective method of control. The space can be purged of
dangerous atmospheres by blowing enough fresh air in, and/or by removing (or suction-venting) the bad air and allowing clean air in. Studies have shown that the best results are obtained by blowing fresh air into a space close to the bottom. Check the efficiency of ventilation by re-testing the atmosphere with the gas detection equipment before entry.
When ventilation is used to improve the air in a confined space, ensure that the toxic or flammable gases or vapours removed from the space do not pose a risk to other workers. “Exhaust air” should not be discharged into another work area.
If you use mechanical ventilation to maintain acceptable atmospheric levels by providing a continuous supply of fresh air, you must have a warning system (I.e., an alarm) and exit procedure in case there is a ventilation failure. The alarm should be activated by a pressure switch at the fan rather than by electrical failure. This ensures that the alarm is activated if the fan belt fails.
In cases where the concentration of explosive gas or vapour is higher than the UEL (Upper Explosive Limit), ventilation will bring the concentration down into the “Explosive Range.” This is one reason why you must use only “explosion-proof” fans. These may be specially designed fans powered by electricity or compressed air. Some pneumatic air movers may also be suitable.
For manholes, you can use portable fans. These usually provide around
750-1,000 cubic feet of air per minute.

A typical manhole 10 feet deep and 5 feet in diameter wide contains 196
cubic feet. Blowing in 750 cubic feet per minute should provide an air
change every 15 seconds and easily dilute or displace most dangerous
atmospheres.
Fans capable of moving 5,000 cubic feet per minute are available for use
in larger tanks and vessels.
This type of ventilation may not be adequate in situations where additional toxic or explosive gases or vapours may be generated (e.g., during cleaning and resurfacing tanks or by disturbing sludge and scale).
In the case of welding or other work which generates a localized source of toxic gas, fume, or vapour, an exhaust ventilator can be used to draw out and discharge the hazard in an open area. (See figure below.)
Options must be evaluated by someone who understands the risks of the work being done.