Are there germs in bad smelling air?

Chris - I have been thinking about this and I think there are probably two aspects. On the one hand, you could say no. On the other hand, you could say, yes!

Let me explain why I'm hedging my bets here. First of all, what is a smell? A smell is a molecule of some kind which is in the air that travels into your nose. Its gets to the top of your nose where you have something called an olfactory epithelium.

There, there are nerve cells that have chemical docking stations called receptors that lock onto that molecule. These tell the nervous system, "I recognise this molecule," and therefore, signal to the brain that this smell is present.

Molecules in their own right are not going to harm you, unless they're a toxic molecule, like hydrogen sulphide for example. That smells of rotten eggs and it's also a bit toxic; but a molecule on its own is not going to infect you with something. But it could, on the other hand, have come from a source of infection: if there's a corpse or something rotting nearby and it's pumping out these molecules, if you can smell them, the source of that smell - microorganisms - is going to be quite nearby. So, you should therefore possibly watch out.

At the same time, and this is where my "yes, it could be a threat" answer comes in - it may be possible also for your nose to detect the physical presence of microorganisms directly. Let me explain that and use an example that probably everyone is acquainted with.

The last time there was a sudden rain shower and you went out of your house and you sniffed the air, you must've noticed that wonderful aroma. It's a clean, fresh, earthy aroma in the air. This exists because when the raindrops come down, they hit the soil and liberate from the soil lots of little spores of an organism that lives in the soil. They're spores from a family of bacteria called Actinomycetes.

On the surfaces of these spores there are molecules which will dock with the receptors in your nose and trigger you to smell that smell. So, what you're smelling, which you think is beautiful fresh post-rain air, is actually loads of microbial spores. Therefore, if you can smell these bacteria by detecting parts of these bacteria, you could argue that it's possible that if you can smell a smell, there could be something potentially infectious in that air, and that could potentially hit you and infect you.

So I reckon the answer is that most of the time, a dodgy smell isn't going to be a risk, but it could signal that there's a risk nearby and so, you could walk into a risk if you weren't careful.

Dave - Are there any pathogenic, dangerous, bacteria which have this spore effect?

Chris - Yes, there are loads of them. The other thing is that Norovirus, which is a very tiny particle just 30 nanometres across - one 30,000th of a millimetre - causes diarrhea and vomiting, and winter vomiting disease. In anyone who's infected with Norovirus, every millilitre of what comes out of their body (at either end) contains about 100 million of these virus particles. They are smaller than the particles of smoke that come off a cigarette. So the chances are, if you smell the smell of "the product of someone having a Norovirus episode", some of the particles from that are drifting around in the air and they're certainly on surfaces in the environment. If you breathe them in, the infectious dose is one or two particles, so you're probably going to get it...