Why Gas Stinks – What Causes Gas to Smell: Gas Safety Check – Gassy Smell
Ethyl Mercaptan is what causes the gas to have a gassy smell and why gas stinks.
The fact that gas stinks helps with a gas safety check.
LPG (propane) gas smells (gas stinks) because an odorant – Ethyl Mercaptan – is added to the gas to make it easier to detect a leak.
Natural gas smells like the same aroma, as the same odorant is used.
In their natural state, LPG (Propane and Butane) and Natural Gas (Methane) smell like nothing, as they are all odorless gases and are also referred to as natural gas liquids – NGL.
The distinctive LPG gassy smell that people associate with these gases is actually added to them as a gas safety check measure.
Gas stinks because Ethyl Mercaptan odorant added to make LPG (propane) and natural gas smell like their distinctive aroma.
- This odorant is added to the gas as it leaves the main storage terminals.
- Most people describe the gassy smell (gas smell) as either rotten eggs or rotten cabbage.
- There are certain end products that require odorant free LPG, such as hair spray or deodorant that use LPG as a propellant.
- Some people cannot smell the odorant and must rely on electronic leak sensors.
Let’s look at some more of the details about why gas stinks…
What Does Propane (LPG) & Natural Gas Smell Like?
For many decades, the gaseous fuels industry has added odorants to make LPG and Natural Gas so gas stinks and gas smell like rotten cabbage so that people can detect gas leaks with nothing more than their noses during a gas safety check.
Most people think that LPG and natural gas smell like rotten cabbage or gas stinks like rotten eggs.
Without the addition of a gassy smell (gas smell), leaking gas could collect without being detected.
This would create a dangerous condition that could lead to an explosion or fire.
The strength of the gassy smell (gas smell) that makes gas stink has caused some people to refer to the process of adding the gas smell as “stenching”.
Why Gas Stinks – What Causes Gas to Smell? Odor Additive
What makes gas stink?
Gas stinks because Ethyl Mercaptan, an odorant, is added to make LPG (propane) and natural gas smell.
However, Ethyl Mercaptan is not a gas, it is a chemical oudorant.
Much research has gone into the science of the gas smell (odorant) and Ethyl Mercaptan is almost universally recognised as the best choice.
As a result, it is the most commonly used gassy smell (gas smell) agent.
Gas Stinks Because of Ethyl Mercaptan in LPG – Propane
Ethyl mercaptan in propane is also known as Ethanethiol.
It is a sulfur compound with the chemical formula CH3CH2SH that is a clear liquid with a very strong and distinctive odor or gassy smell.
Ethyl mercaptan is commonly used as a gas smell with natural gas and propane-LPG, as they are naturally odorless.
How & When it Gets Added
In the case of LPG, the Ethyl Mercaptan is added to the gas as it leaves the main storage terminals.
The amount of gassy smell (gas smell) added and the process are both carefully controlled so gas stinks in the right amount.
The terminals themselves have gas detectors that can identify gas leaks without any gassy smell (gas smell) having been added.
Ethyl Mercaptan is Used in LPG Because of its Stability Over Time
Ethyl mercaptan (not methyl mercaptan) is used in LPG as an odorant to generate the telltale gas smell.
Ethyl Mercaptan will maintain the chemical equilibrium across the liquid and vapour space.
However, if the tank liquid level is low, and much of the tank surface inside is exposed, then a small amount of gas smell (odorant) fade MIGHT occur, reducing the overall concentration of the Ethyl Mercaptan in both liquid and vapour LPG.
Given the detectability of the gassy smell (gas smell) is much less than the dosing used in Australia (we can detect the gas smell in parts per billion, and we dose in parts per million), the small amount of fade should not be an issue.
Methyl Mercaptan Used in LPG
Methyl mercaptan is not used in LPG.
People get confused between Ethyl Mercaptan, which is used as the gassy smell (gas smell) in LPG, and Methyl Mercaptan, which is not used in LPG.
Methyl mercaptan reportedly used as a dietary supplement for animals.
How to Test for Leaks
Special Cases of Gas with No Gas Smell
There are certain gas applications where the gassy smell (gas smell) is not added.
Facilities that use odorless gas must have the same gas detection equipment as the gas terminals.
For example, Butane is commonly used as an aerosol propellant.
Needless to say, we wouldn’t want things like hair spray and deodorant to have a gassy smell like rotten cabbage!
Why do LPG (Propane) Cylinders Have Greater Gas Smell When Near Empty?
The Ethyl Mercaptan gassy smell (odorant) is slightly less volatile than the Propane in which it is dissolved.
As the Propane vapour is used by the appliance, the gassy smell (gas smell) is also removed in the process.
Due to its lower volatility, there is a slightly higher concentration of odorant as the last of the Propane liquid is converted to vapour and used.
The gassy smell (gas smell) is noticeable at very low concentrations (by design), so this small increase in concentration can be very apparent.
Some People Can’t Detect the LPG or Natural Gas Smell
Some people cannot detect the LPG or natural gas smell.
This can be due to illness or if they have been continuously exposed to the gassy smell.
If you know that you can’t detect the gassy smell (gas smell), have problems with your sense of smell or just want an extra layer of protection, you might consider installing a gas detector.
Gas detectors emit an audio alarm when gas is detected, similar to a smoke alarm.
Ethyl Mercaptan Trivia
Whilst some people cannot smell Ethyl Mercaptan, evidently FLYS can!
We are told by Mackay Queensland gas fitter Josh Dalton:
“FLYS love the smell of ethyl mercaptan. If you see a bunch of fly’s buzzing around your Meyer, cylinder, pipe work, regulators, fittings or appliances, there may be good chance there is a leak.
It’s one of the first things I look for automatically.”
Thanks for the tip, Josh.
To explain, some species of flies are attracted to the Sulphur in the Ethyl Mercaptan and will buzz around like bees.
The amount of Ethyl Mercaptan required is very small, so even a very small leak will attract some flies in the vicinity.
Gas Smell Fade
Ethyl Mercaptan is not a perfect odorant.
No odorant can provide an absolute guarantee of no gas smell (odorant) fade.
Under some circumstances, gas smell can fade away and be replaced by a gentler smelling odor that might not be recognised as a gas leak.
Fade is caused by adsorption, absorption and oxidation.
Adsorption involves the gas smell (odorant) ‘sticking’ to the inside steel walls of the gas bottle.
Absorption refers to the gassy smell (gas smell) being absorbed by another substance such as water.
Oxidation occurs when the inside of a gas bottle is exposed to air, causing a chemical reaction which can result in LPG losing its gassy smell.
Gas smell (odorant) fade is rare but it can happen.
While very few instances of gas smell (odorant) fade have been recorded in Australia, it has happened in other countries.
The presence of rust or moisture within an LPG tank could cause this gas smell (odorant) fade.
To prevent this, new cylinders are filled with dry and inert nitrogen gas, to prevent both rust and eliminate the presence of moisture.
Once filled with LPG, the risk is virtually eliminated.
What Suppliers do to Prevent Gas Smell Fade
- Suppliers make sure gas bottles and tanks are properly conditioned prior to filling.
- Suppliers ensure the correct amount of odorant is added to LPG.
- Then they monitor the levels of gas smell (odorant) at all points of the supply chain.
- They make sure gas storage tanks and gas bottles are clean inside and out.
- Gas bottles and tanks are purged of air, water and other substances prior to use.
What You Can Do
LPG users can also assist in avoiding gas smell (odorant) fade by making sure that all disconnected gas cylinders have their valves closed, even when completely empty, to stop air (oxygen) and moisture from getting inside the cylinder.
This helps prevent the possibility of internal rusting and fade by oxidation.
Rust and moisture are also one of the things that are looked for when gas cylinders are periodically re-inspected.
The presence of either is cause for condemnation of the cylinder.
You can also follow the these tips:
- Do not store your gas bottles indoors or in an enclosed space, in case of a leak.
- Do not use any gas bottle, regulator, hose or fitting that is damaged.
- Only buy gas bottles, regulators, hoses and fittings from reputable suppliers.
So, now you know why gas stinks the way it does and why it is the ‘Gas Smell of Safety’.
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Steve Reynolds
Technical Consultant
Steve Reynolds is a leading expert in the LPG industry with over 22 years of experience. As part of the national management team at ELGAS, Steve ensures the safe and efficient storage, handling, and transportation of LPG. He serves as the lead investigator for incidents and collaborates with authorities on industry developments.
Steve is a technical advisor to Standards Australia and Gas Energy Australia (GEA), and an active member of the World LPG Association (WLPGA), contributing to global standards and technical reviews. He holds a BSc. (Hons) in Industrial Chemistry from UNSW and has held senior safety and technical roles at ELGAS, making him a trusted authority in LPG safety and standards.