The Hidden Danger Inside Manholes
- Karen Scheepers
- 5 hours ago
- 4 min read
Manholes and sewer openings are a common sight in communities across South Africa. Most people pass by them without a second thought. But what many don’t realise is that these access points can conceal a lethal threat: sewer gas. Invisible, fast-acting, and often undetectable by smell alone, this toxic mixture can overpower and kill within seconds.
Understanding the makeup and dangers of sewer gas is critical for public safety. It’s not only maintenance workers who are at risk, curious residents, good Samaritans, or anyone near an open or blocked manhole may find themselves in danger if the proper precautions are not taken.
What Gases Are Found in Sewers?
Sewer gas is not a single substance, but a dangerous cocktail of several gases produced as organic waste decomposes. Chief among these is hydrogen sulfide (H₂S), a gas that smells like rotten eggs at low levels but becomes undetectable, and far more deadly, at high concentrations. Even a few breaths can cause dizziness, unconsciousness, or death.
Another common component is methane (CH₄). This gas is odourless and flammable. While not toxic in the same way as hydrogen sulfide, it displaces oxygen and poses a major explosion risk. High levels of methane in a confined space can lead to suffocation without warning.
Also present are carbon dioxide (CO₂) and carbon monoxide (CO). CO₂, while not poisonous on its own, can push oxygen out of the air and cause suffocation. CO, on the other hand, binds to the blood’s oxygen-carrying molecules and can be fatal even in small quantities.
Ammonia and a wide array of volatile organic compounds (VOCs) further contribute to the mix, irritating the eyes and respiratory system, and making an already hazardous atmosphere even more dangerous.
Why Sewer Gas Is So Dangerous
What makes sewer gas so deadly is not just its contents, but how quickly it can affect the human body, and how little warning you may get.
Many people assume that if they can’t smell anything unusual, the air must be safe. This is a dangerous misconception. Hydrogen sulfide, for instance, can numb the sense of smell at high levels. What might start as a strong odour can disappear suddenly, not because the gas is gone, but because it's reached a concentration that overwhelms your senses.
Similarly, methane and carbon monoxide have no smell at all. There are no clear visual or olfactory clues to warn you when you’re breathing something that could kill you in seconds.
Symptoms of exposure, such as nausea, headache, eye irritation, dizziness, or confusion, can appear within moments. But in high concentrations, sewer gas can cause a person to collapse instantly, without any time to react. Victims may simply fall unconscious where they stand.
The danger is especially acute in manholes and other confined spaces. These areas often lack airflow, allowing gases to collect undisturbed. Because some of these gases are heavier than air, they settle at the bottom, making even a quick descent into a manhole life-threatening.
Key Safety Tips for Residents
The most important rule: never open or enter a manhole. These structures are not meant for public access and can hide lethal atmospheres. Even leaning into an open manhole to inspect or smell can result in immediate collapse from toxic exposure.
If you see a manhole overflowing, unsealed, or blocked, do not attempt to fix it yourself. Don’t poke around with a stick or open the cover to relieve pressure. It is not just raw sewage that poses a risk, it’s the invisible gases released by the decaying waste.
If someone appears to be in distress or has collapsed near a manhole, do not attempt a rescue. This is where many additional deaths occur: one person collapses, and others rush in to help, unaware that they are walking into the same poisoned air. Most deaths in these incidents are not of the first person affected, but of those who try to help without protection.
Instead, alert emergency services immediately. Provide clear details of the location and report a suspected toxic gas or confined space hazard. Keep bystanders away and, if possible, move upwind of the opening.
Blocked manholes and leaking sewers should be reported promptly to local authorities. It’s helpful to take photos from a safe distance, note the time and location, and encourage others in your area to report the issue as well. Repeated complaints often receive faster attention.
Preventing Tragedy Through Vigilance
While residents must take care never to enter or tamper with manholes, there’s also a broader need for awareness and accountability. Blocked sewers and long-neglected infrastructure are more than just an inconvenience, they’re a public health threat.
When manholes remain clogged for days or weeks, pressure can build up inside, increasing the risk of a sudden release of toxic gas. Communities need prompt response and regular maintenance to ensure that these risks are kept in check.
Just as importantly, anyone tasked with working in or around sewers should be properly trained and equipped. This includes testing the air in a manhole before opening it, using ventilators to disperse trapped gases, and wearing breathing equipment if entry is absolutely necessary. Unfortunately, some tragedies have occurred because proper safety procedures were ignored, skipped, or unavailable.
As a community, asking questions, raising concerns, and demanding action when blocked drains or open manholes are left unattended can make a difference. Public pressure can ensure that safety protocols are followed and infrastructure is maintained before disaster strikes.
A Tragedy That Should Never Be Repeated
On 15 October 2025, two residents lost their lives and another was hospitalised after being exposed to sewer gas from a blocked manhole. It happened after attempts were made to clear the obstruction without proper equipment or support. The gas overwhelmed them almost instantly, and only one person made it out alive.
This incident, which unfolded in a quiet residential area, is a heartbreaking example of what can go wrong when the silent danger inside manholes is underestimated. It serves as a sobering reminder that the risks are real, and that caution, awareness, and proper reporting are not just bureaucratic steps, but potentially lifesaving actions.
Let this serve as a warning and a call to action. The gases lurking beneath our feet may be invisible, but their impact is devastatingly real. By staying informed and alert, residents can help protect not only their own lives but the safety of their entire community.
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