Air Quality in Confined Spaces: Why Assumptions Are Dangerous Hazards

Air quality in confined spaces can change in an instant—and assumptions can be costly. Here’s why continuous monitoring and critical thinking must guide every entry.

• By Tom Burgess
• Jun 30, 2025

When it comes to safety in confined spaces, air quality is recognized as one of the most critical elements, but the hazards can still be sometimes misunderstood. There are many factors that can affect atmospheric conditions inside a confined space before and during entry. 

Going Beyond a Multigas Reading 

Multigas meters to measure oxygen levels, flammability, and hydrogen sulfide and carbon monoxide concentrations are commonly used in industry and for confined space entry. These may or may not adequately detect and measure the hazards in the space. Even when they do, for example an LEL meter will detect flammable solvent vapors, it may not provide adequate protection. 

For example, the lower explosive limit (LEL) of methyl ethyl ketone is 1.8 percent. A LEL meter is typically set to alarm at 10 percent LEL, which for methyl ethyl ketone is a 0.18 percent concentration in air. 1.0 percent is equal to 10,000 parts per million (ppm), so a 0.18 percent concentration of methyl ethyl ketone is 1800 ppm. This is much higher than the OSHA permissible exposure limit of 200 ppm and the recommended ACGIH threshold limit value of 75 ppm. The LEL meter protects from flammability does not provide warning for the exposure limits intended to protect worker health.   

Also, a multigas meter may not detect all the potential atmospheric hazards. A photoionization detector (PID) or chemical specific sensors are among the options to provide measurement in the ppm range for comparison with exposure limits and detect a broader range of contaminants.  

Real-World Risks: Atmospheric Hazards from Unlikely Sources 

One type of atmospheric hazards that I find commonly underestimated are hazards from natural and man-made sources in underground spaces. We commonly look at hazards from sources within a space, but underground spaces may be impacted by underground conditions or sources around the space that we may not even be aware of. 

It would be easy to assume that something like an empty underground vault could not have an atmospheric hazard. I always look suspiciously at any unventilated underground space. Perhaps there is a natural source of methane or other gases that has slowly seeped into the space, displacing oxygen or creating a hazardous atmosphere. Maybe there is some manmade source like underground piping or tanks that leak with gases or vapors seeping into the space. 

Underground conditions may also change over time, so hazards may arise in spaces that never had a hazardous atmosphere before. These are exactly the situations where an experienced worker may assume that they don’t need air monitoring because their experience with the space has led them to assume that it will always be safe to enter. 

I have done extensive work with water and wastewater systems assessing water and water systems for clients. One project I was involved with included inspecting hundreds of manholes and vaults used for a large public water supply system. These were the type of locations that you may not expect a hazard, only water pipes, maybe some pumps, usually pretty clean and dry.  

And in most of the spaces the air quality was fine, but every now and then there would be oxygen deficiency or other issue. Sometimes from an identifiable source like water and organic debris, other times the source of the hazard was not clearly evident. After testing numerous spaces and finding them safe, we may want to assume that the next one or the next time we enter the same space is fine too.  

Temperature, season, or animal activity can also change atmospheric conditions. Warmer summer months promote natural degradation and biological activity, which increases the risk of hydrogen sulfide or methane building up in poorly ventilated spaces. Colder winter months may suppress these reactions, lulling workers into a false sense of security. It doesn’t matter whether the space was safe yesterday; what matters is whether it is safe to enter right now. 

Common and Uncommon Hazards 

Hydrogen sulfide is particularly sinister since the poisonous gas has an odor at low concentrations but at higher levels, it paralyzes the ability to smell, leaving workers unaware of its presence. The colorless gas is also heavier than air making so the concentration can increase the lower you go into the space. Unfortunately, there are numerous fatality cases of workers entering an underground space only to descend into increasing concentrations of the invisible odorless poisonous gas. Hydrogen sulfide can be produced by natural organic decomposition within a space, or in the surrounding underground environment. 

Carbon monoxide is another colorless, odorless poisonous gas that we normally associate with gas engines, heaters and other sources of combustion. In underground spaces a source of carbon monoxide can be electrical equipment and underground power lines. Overheating of electrical equipment can result in burning or decomposition of electrical insulation releasing smoke and variety of gases including carbon monoxide. Smoke and odor can provide a good warning that something is not right in our space, but those indicators may not always be reliable. 

Methane is a flammable gas that can be produced from natural underground sources. Methane is much lighter than air, so it usually rises and dissipates quickly, but if an underground space is unventilated methane may accumulate and may also displace oxygen. 

Monitoring, Interpreting, and Responding 

Staying on top of air quality monitoring requires more than routine gas meter checks. It involves a structured approach grounded in the right instrumentation and understanding of what’s being measured. Consider the following: 

• It’s important to choose meters with specialized sensors that detect key hazards, such as flammable solvents or corrosive gases. Standard multi-gas meters don’t everything. 

• Conditions can change while work is ongoing. Continuous monitor conditions. Modern wireless or Bluetooth-enabled devices make it easy for to remotely monitor air quality in real-time. 

• Ventilation is often the first corrective action for hazardous atmosphere, but there’s planning needed to make sure that it is effective.  Mechanical or natural ventilation must be sufficient to disperse flammable or toxic vapors and restore oxygen levels from start to finish.  

• Pre-entry assessments must consider internal and external sources of contaminants, such as those originating from nearby sewers, fuel lines, or decaying organic matter.  

A Different Kind of Hazard Recognition 

When it comes to air quality in confined spaces, the most dangerous thing you can do is assume a space is safe simply because it was previously. Remember: past safety does not guarantee future safety. Every confined space must be treated as a dynamic environment. It may look safe, but that doesn’t mean it is safe. And when the space becomes unsafe, can your team be prepared to respond immediately? 

When preparing to enter a confined space, anticipate every possible scenario. That’s because “It’s probably fine” is never an acceptable safety standard in the world of confined spaces. 

This article originally appeared in the June 2025 issue of Occupational Health & Safety.