What Causes Bad Smells?

On 720 ABC today the topic of discussion was “what’s the worst thing you have spilled in your car?”

It led to a catalogue of calamities and catastrophes involving aftershave, butter, steak, and even a dead body.

So what causes bad smells and what can we do about them?

Essentially, bad smells are caused by three classes of chemicals – volatile fatty acids (VFA), amines and mercaptans.

The VFAs are any dairy product, body odour, or vomit.

Amines are urine and fishy smells.

Mercaptans are anything that is rotten, although these can also contain amines, like the elegantly named putrescine and cadaverine.

As it happens the VFAs and amines are relatively easy to deal with, but the mercaptans are a whole different ball game.

With the VFA’s the relevant word is “acid”. Because they are acids, their volatility, and therefore smell, may be counteracted by simply neutralising them with an alkaline product of some sort. In this case, ammonia would be the product of choice. The neutralised fatty acid (for for example, ammonium butyrate) could then simply be blotted up with a paper towel. Make up a solution of ammonia, about one part in 20, spray it on, then just blot it up. You will find after this that the smell has pretty much disappeared.

With amines, exactly the opposite approach is required. Because amines are alkaline, the choice of a cleaning agent is an acid. The vinegar is an excellent choice because it has a smell of its own, and no one mines a residual smell of vinegar after you have cleaned something.

So if your cat has weed on your carpet, spray some vinegar on it, leave it for a little while, then blot it up. Same with a fishy smell.

The effectiveness of these two approaches is the result of simply working with the chemistry of the products in question.

But with mercaptans, however, this approach is not available to us, as mercaptans have three properties that all conspire against us:

1. They are chemically stable and are not easily reacted with anything to bring about a change in their chemistry which would change their smell.

2. Being sulphur based compounds they are very chemically sticky. That is, the non-bonding electron groups in the sulphur will bind strongly to many substrates (this is the reason that sulphur-containing natural fibres like wool and cotton stain more easily than synthetic fibres).

3. Mercaptans also have the property of having are very intense smell. That is, a lot less mercaptan is required to create a bad smell than is required for the VFA’s or amines. Typically, a mercaptan only has to be present in parts per billion quantities to produce a noticeable smell. An example here is rotten egg gas (H2S).

H2S is actually more toxic than cyanide gas (the stuff used by the Nazis in their gas chambers). This may surprise you, as we have all smelled it and are still alive. The reason is simple – it has an incredibly intense smell and is detectable by our noses in tiny, tiny amounts.

This property can, however, be put to good use. Mercaptans are added to natural gas and LPG (which are otherwise odourless) in tiny amounts so that if there is a leak we can smell it.

So the lesson is this – VFAs or amines are relatively easily to remove, but if you’ve had a rotten steak or dead rat in your car, it may well be impossible to remove the smell. If you can get to the surface you could try dislodging it with either a caustic cleaner (Easy Off Oven Cleaner) or a solvent (acetone), but given that most things in your car are porous (including plastic) it may be a lost cause.

Mythbusters discovered this with a dead pig smell in a car. No one could remove the smell even with the interior completely stripped. Some things just can’t be done….



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