Do candles really clear nasty smells?

Patrick sent us this question: "My wife wants me to light a candle after doing my number two, to get rid of the smell. Does this actually do anything?"

Phil Sansom has been sniffing out an answer, with the help of Kit Chapman from Chemistry World.

Patrick - My wife wants me to light a candle after doing a number two to cover up the smell. Does this actually do anything? I'd love to find out!

Phil - Not exactly our first choice of question - on our list it was number two - but the Naked Scientists take these things seriously - so I will not back down from a journalistic challenge. I will tackle this new frontier in research, and I will do it the only way I know how, and that’s with a hard-hitting in-depth personal investigation. I will go out and do a poo myself and record it - for science.
First, I just need to OK this with the rest of the Naked Scientists team.

Izzie - No!
Adam - No! Why? No!
Ruby - That’s...really gross.
Ben - Phil! What? Of course not!

Phil - Huh. OK. Well what do they know. My boss Chris has the final say. Surely he won’t stand in the way of my duty?

Chris - No, please don’t do that Phil. Please don’t do that.

Phil - Ok, umm...I guess I need to find another way to answer this question. Who do I know who would be able to explain the effects of a candle on those nasty toilet smells?
How about Kit Chapman, the comment editor for the magazine Chemistry World? He can help me out. So Kit: isn’t it just that the candle burns away all those gases, the ones from your rear end that make the smell?

Kit - Sadly, you can’t just burn away the smell of your latest poo. People might think that, based on online videos mixing fire and flatulence - and a quick check produces a whole host of people creating spectacular flames from their farts. It works because hydrogen, hydrogen sulfide and methane are flammable. But this isn’t recommended – you can get seriously hurt - and it doesn’t tackle the smell problem. Of the three gases only the hydrogen sulfide gives off an odour, and usually that tell-tale stink from cutting the cheese is caused by a host of other chemicals produced by gut bacteria.

It’s also important to remember breaking wind provides a relatively high concentration of these flammable gases before they are quickly dispersed in the atmosphere. A good thing, too, otherwise a romantic candlelit dinner at a curry house would probably end in third degree burns.

So a candle flame can’t burn off the pong of your number two. Your toilet deposit isn’t a jet of concentrated gas, so it won’t cause ignition, and its whiffiness is caused by chemicals that aren’t known for spectacular combustion.

Phil - Wow, OK - so it’s not that simple. The gases that catch fire aren’t the same as the chemicals that make the bad smell. So that’s one theory out the window. Does this mean it’s pointless to light a candle? More importantly is Patrick wasting precious, precious candles for nothing?

Kit: But this doesn’t mean Patrick’s wife is wrong. If it’s a scented candle or a melt, burning it will mask odours and give the bathroom a pleasant aroma. It’s because they contain essential oils or other fragrances in their wax. When the candle is burned, the wax melts and the oils evaporate, filling the air with volatile organic compounds such as limonene, found in citrus fruit scents; linalool in lavender; or pinene in pine. Any of these scents will react with the receptors in your nose, masking the nasty funk from your faeces.

But do bear in mind that scented candles have been found to cause indoor air pollution - especially ones that have paraffin wax, they tend to release carcinogens into the air; and ones that have wicks wrapped around some sort of metal, they can release toxic soot. 

Phil - So your choice of candle could make all the difference when it comes to how stinky your toilet seems for the next visitor.
But even if the candle itself isn’t scented, the match you use to light it will be! Or at least it will be for a second. There’s a lot of sulfur in match heads, and when you burn off that sulphur you make sulfur dioxide. That’s a really pungent gas that your nose’s smell receptors are particularly sensitive to.

Finally Vaniar says via the webform: there’s a real solution that eliminates all smell, but it is not a candle. Use equal parts hydrogen peroxide and rubbing alcohol. They can be poured in to the toilet straight from the bottles - try two caps full of each.” So there’s something else that might work.

Thanks for your help Kit, you really put us on the scent of the right answer. Now we can flush it from our minds. Patrick, your wife’s right, keep doing what you’re doing. And next week we’re answering this question:

Is it possible to have so many blood transfusions that your blood type changes?