What is a common safety precaution before entering a manhole?

Ensuring adequate ventilation is a crucial safety precaution before entering a manhole because it helps to prevent the accumulation of hazardous gases that can develop in confined spaces such as sewers. These gases, which may include methane, hydrogen sulfide, and others, can be toxic or even flammable. Proper ventilation allows for a safer working environment by diluting these gases and reducing the risk of asphyxiation or explosion.

In practice, this can involve using fans or blowers to introduce fresh air into the manhole or monitoring the air quality with appropriate testing equipment before entry. This step is vital for protecting the health and safety of workers entering potentially hazardous and confined environments. Other options may address specific site conditions or situations but do not directly enhance the immediate safety of the worker in terms of air quality and gas presence.