If bad air is expected, OSHA says there must be a rescue plan and rescue equipment on the job site. Rescue teams must have special training. The best way is to follow OSHA rules for rescues from confined spaces. Check the air for toxics like chlorine, carbon monoxide, sewer gases, and hydrogen sulfide. These toxics can kill.
Carbon monoxide has no smell.
Hydrogen sulfide smells like rotten eggs. The competent person will decide if blowers can keep the air safe. A competent person must check the soil. This check helps the competent person choose the right worker-protection system. A trench can be in stable rock, or type A, type B, or type C soil.