Gas hazards in sewers: If the flame of a miner’s safety lamp in the upper layers of a sewer atmosphere forms an explosive condition, which gas is most strongly implicated?

Introduction / Context:
Explosive atmospheres in sewers are serious hazards. Recognizing which gas is the principal cause helps enforce ventilation and monitoring. Methane, produced by anaerobic decomposition, is the classic flammable constituent in sewer headspaces.

Given Data / Assumptions:

• Upper layers of a sewer, where lighter-than-air gases may collect.
• Observation of an explosive response with a flame (historical indicator).

Concept / Approach:
Methane is a combustible gas with lower and upper explosive limits in air. In oxygen-deficient conditions methane may not ignite; however, when mixed in explosive proportions with sufficient oxygen, it can form an explosive atmosphere. CO2 is asphyxiating but non-combustible; H2S is toxic and flammable but less commonly the primary driver of widespread explosivity compared with CH4 in sewers.

Step-by-Step Solution:
Identify the gas associated with explosive atmospheres in sewers: methane.Recall that sewage generates CH4 under anaerobic digestion conditions.Select methane as the correct answer.

Verification / Alternative check:
Confined space entry procedures require monitoring of %LEL (lower explosive limit), typically linked to methane for sewer systems, alongside O2 and H2S monitoring.

Why Other Options Are Wrong:
CO2: Non-flammable; cannot create explosive mixtures.H2S: Flammable but usually present at lower concentrations; the more universal explosive concern is methane.O2: Not a fuel; it supports combustion but is not itself explosive here.

Common Pitfalls:

• Confusing asphyxiation risk (CO2, low O2) with explosivity risk (CH4).
• Relying on flame tests instead of mandated calibrated gas monitors.

Final Answer:
Methane (CH4)