Sewer gas deaths: the science behind fatalities and protective gear

In July 2024, the Government informed the Rajya Sabha that 377 people had died between 2019 and 2023, while they were involved in the hazardous task of cleaning sewer and septic tanks. Just a few years ago in Chennai, at a prominent mall, a worker lost his life while cleaning a septic tank — and this is despite manual scavenging being banned in the country. Ironically, two national surveys (in 2013 and 2018) claimed that manual scavenging no longer existed in India. The challenge therefore, lies in identifying manual scavengers, as their work is often informal employment or outsourced contracts. 

While there is undeniable sociopathic negligence with regard to this practice, the conversation often focuses solely on its sociological aspects. It is also critical however, to understand how sewer gases are formed, their impact on the human body, and the mechanisms of fatality and consequences of exposure to these gases.

What is sewer gas?

Sewer gas is a generic name for a mixture of toxic and non-toxic gases. Sewer gases are formed through the microbial decomposition of organic matter in anaerobic or oxygen-deprived environments. Organic waste serves as the fuel for bacterial activity, breaking down into harmful gases such as hydrogen sulfide (H₂S), methane (CH₄), ammonia (NH₃), carbon dioxide (CO₂), and carbon monoxide (CO). H₂S is produced when sulphate-reducing bacteria break down sulphates in organic matter. Methanogenic bacteria generate methane and are odourless but highly flammable, posing a risk of explosions. Ammonia is formed from nitrogenous waste and can cause severe irritation to the respiratory system. Carbon dioxide and carbon monoxide arise from organic matter degradation and incomplete combustion.

H₂S is the most hazardous sewer gas. In nature, it originates from volcanic eruptions, hot springs, and the decay of organic matter in environments like swamps, marshes, and coal pits. Industrially, H₂S is prevalent in sectors such as oil and gas refining, mining, tanning, pulp and paper processing, rayon manufacturing, and wastewater treatment. The formation of sewer gases, especially H₂S, depends on a high concentration of organic waste, long and thick sewer pipes with little airflow, stagnant water, acidic environments and warm temperatures. 

H₂S is infamous for its distinct rotten egg odour, a warning signal at low concentrations (<0.03 ppm). However, this warning mechanism fails due to olfactory fatigue at dangerously high levels (>100 ppm). When a human being is exposed to elevated concentrations of H₂S, the gas paralyses the olfactory nerve, disabling a person’s ability to detect its smell. This is why workers, even if initially alerted by the odour, may suddenly stop noticing it as they inhale more of the gas.

The effect of H₂S on humans

In sewer pipelines, the H₂S concentration can vary depending on the conditions. However, levels capable of causing death are common in poorly-ventilated and stagnant systems. H₂S concentrations in such environments can range from 100 ppm to over 1000 ppm. At concentrations above 100 ppm, the gas, apart from paralysing the olfactory nerve, can cause severe eye and respiratory irritation. When levels exceed 500 ppm, exposure can result in rapid loss of consciousness and respiratory distress within minutes. At concentrations greater than 1000 ppm, H₂S can lead to immediate respiratory arrest and death due to its ability to block cellular oxygen utilisation. 

At high concentrations, the gas disrupts cellular respiration by inhibiting enzymes in the cytochrome oxidase pathway, depriving cells of oxygen despite the body’s ability to breathe. This can lead to dizziness, loss of consciousness, and collapse within moments. For workers exposed to such high levels, the progression from alertness to unconsciousness is rapid, resulting in death. 

Imagine your body as a car, with oxygen as the petrol powering its engine. Inside the car, the fuel system is self-contained—the tank (lungs) have the petrol and pipelines (blood vessels) deliver it. The engine (cells) use it to produce energy through an ignition system. The cytochrome oxidase pathway acts like this ignition, helping cells use oxygen effectively. When hydrogen sulfide enters the body, it clogs this ignition system, preventing the engine from burning the fuel. Even though the car has petrol, it can’t run. Hydrogen sulfide stops cells from using oxygen, causing the body to “stall“ and shut down.

At low concentrations, hydrogen sulfide and ammonia irritate the respiratory system, causing symptoms like coughing, wheezing, and strained breathing. Prolonged exposure, even at low levels, can lead to long-term lung damage, putting workers at risk of chronic respiratory illnesses. H₂S and carbon monoxide also affect the nervous system, causing headaches, dizziness, confusion, and, over time, issues like memory loss and cognitive decline. Ammonia and other acidic substances in sewer gases irritate the eyes, leading to redness, watering, and discomfort. Meanwhile, prolonged skin contact with contaminated sewer water often causes rashes and dermatitis. 

The importance of safety gear

Many sanitation workers in South Asia enter poorly-ventilated sewer systems without adequate training or protective equipment. In developed nations, stringent safety measures protect sewer workers from exposure to toxic gases. Advanced ventilation systems ensure fresh air circulation, minimising the buildup of hazardous gases. Portable multi-gas detectors monitor air quality in real-time, alerting workers to unsafe conditions. Workers are equipped with self-contained breathing apparatuses (SCBA) and chemical-resistant suits to protect them against gas exposure. Robotics and automation have further transformed sewer maintenance.

In the movie The Shawshank Redemption, Andy Dufresne crawled through 500 yards of sewage to escape from a U.S. prison in the 1940s. That scene set nearly 80 years ago, portrayed the triumph of resilience and hope as Andy emerged alive, unscathed, and free to rebuild his life. In the United States, even then, sewer systems weren’t the deadly gas chambers they could have been. Fast forward to 2024, and the reality in South Asia is depressing. Even today, sewer lines in this part of the world remain death traps, filled with lethal gases like hydrogen sulfide, turning them into execution chambers. Workers sent into these pipelines don’t crawl toward freedom like Dufresne did, but are pulled out as lifeless bodies, victims of an inhumane system that has failed them. 

(Dr. C. Aravinda is an academic and public health physician. The views expressed are personal. aravindaaiimsjr10@hotmail.com)