The short answer
Septic tanks give off methane, carbon dioxide, hydrogen sulphide and some ammonia as bacteria break down the sludge. Normally these gases leave through the house's soil vent pipe and do no harm. Inside the tank or a chamber they can build up to dangerous levels, which is why nobody should ever lean into or enter a septic tank.
This page is about safety. For tracking down an everyday smell, see why does my septic tank smell.
Which gases are produced?
| Gas | Where it comes from | The risk |
|---|---|---|
| Methane | Breakdown of organic matter without oxygen | Flammable and can be explosive in an enclosed space. Displaces oxygen |
| Carbon dioxide | Same process | Heavier than air, so it collects low down and pushes out oxygen |
| Hydrogen sulphide | Bacteria breaking down sulphur compounds in waste | Toxic. Smells of rotten eggs at low levels, but can stop you smelling it at high levels |
| Ammonia | Mainly from urine | Irritating to eyes and lungs in a confined space |
Why hydrogen sulphide is the one to worry about
At low concentrations hydrogen sulphide is easy to notice. As levels rise it deadens the sense of smell, so the warning disappears at the moment the gas becomes most dangerous. It is heavier than air, so it settles in tanks, chambers and trenches. People have died entering sewage tanks to retrieve something or clear a blockage, and would-be rescuers have died going in after them.
The rule is simple. Never put your head into an open tank, never climb into one, and never go into a chamber or deep excavation near one. Work inside a tank is confined space work, for trained people with gas monitors and breathing equipment.
How venting keeps gas where it belongs
A septic tank is vented through the drains. Gas rises back up the pipe from the tank and leaves through the soil vent pipe on the house, which should end above the eaves and away from windows. Approved Document H asks for septic tanks to be ventilated with the ventilation kept away from buildings.
If the vent is blocked, cut short or replaced with an air admittance valve on every stack, the gas has nowhere to go. It may escape around the tank lid or force its way back through traps into the house.
Covers and lids
Building regulations expect access covers on a septic tank to be lockable or otherwise engineered to prevent anyone getting in. Old concrete slabs and rotted timber lids are a real hazard, particularly with children or animals about. Replacing them is quick and inexpensive.
Signs gas is escaping where it should not
- A rotten-egg smell near the tank, the drainage field or inside the house.
- Gurgling drains, which can point to a venting problem.
- Corroded metal fittings or crumbling concrete around the tank opening. Hydrogen sulphide forms acid on damp surfaces.
Staying safe around a septic tank
- Keep covers closed and secure, and don't stand on old or cracked lids.
- Keep naked flames and cigarettes away when a tank is open.
- Leave opening and inspecting the tank to the emptying contractor or an engineer.
- If someone collapses near or in a tank, do not go in after them. Call 999 and say it may be sewer gas.
Does gas mean something is wrong with the tank?
Producing gas is normal, though you should not be smelling it where you live and breathe. The cause is usually a venting fault, a poor seal, an overdue tank or a failing drainage field, and each has a fix.
Our septic tank inspections check covers, venting and the tank's condition safely from outside. We replace unsafe lids and sort venting problems as part of our septic tank repairs.
Ventilation and cover requirements from Approved Document H. Checked on 15 October 2026.
