[Make it Lady (OP)]

I sat next to a lady the other day. Her breath smelt very bad. I found myself holding my breath just to make sure it wasn't mine but the wofts were definitely hers. It set me thinking about why we sometimes get bad breath. My husband always has it after long flights even if he cleans his teeth. My friend had bad breath on a particular diet. My bad breath moments are usually accompanied by a hangover. So what are the main reasons?

[Chemistry4me]

USUALLY bad breath is caused by bacteria that live in your mouth, and especially on the back of your tongue.  there are millions of these, poor oral hygeine and/or gum infection can increase these which cause the smell

if the smell is coming from your lungs you better worry and see a doc because it could be a chest infection, or depending on what kind of smell - can signify other serious problems like diabetes (if you are diabetic and end up with hyperglycaemia, you excrete ketones in your breath which have an odd smell which is often described as "pear drops" smell....sinus, liver and kidney disease can also be causative factors of bad or odd smelling breath...

on saying that, the most common thing is bacteria - on the tongue, around the teeth, plaque, and the fact these bacteria can munch sugars and turn them into acid..smelly.

http://www.thenakedscientists.com/forum/index.php?topic=12553.0

[Make it Lady (OP)]

My dieting friend smelled of ketones!

[Chemistry4me]

Is she a diabetic? What is "pear drops" smell, fortunately I have never come across it. Or maybe I have, just did not realise...

[Make it Lady (OP)]

It is a sweet sickly smell.

My friend isn't diabetic.

[Karen W.]

 decaying or bad teeth or tooth causes bad breath.. a pice of meat stuck in a tooth and not brushed free will cause terrible odorous breath!!!

[RD]

What is "pear drops" smell

Ketone "pear drops" breath also smells like nail varnish remover / paint thinner, (acetone).

[Make it Lady (OP)]

Yes but out of the mouth of your best friend it always smells worse than nail varnish remover.

[nogood]

There are various causes for bad breath or halitosis. The most common is tooth decay or the lack of proper dental hygiene. Smoking can also cause bad breath along with a person's diet. IT could also be that the person has some sort of illness that causes her breath to emit such odor...

[chris]

The usual culprits behind radioactive breath are bacteria. Marcello Riggio, from Glasgow University, has been looking at this question and joined us on the Naked Scientists podcast to talk about it:

http://www.thenakedscientists.com/HTML/content/interviews/interview/914/

Chris