[00:03.12]2005

[00:06.89]The human nose is an underrated tool.

[00:09.60]Humans are often thought to be insensitive smellers

[00:12.53]compared with animals,

[00:14.04](1)but this is largely because, (2)unlike animals,

[00:16.97]we stand upright.

[00:18.69]This means that our noses are (3)limited to

[00:20.81]perceiving those smells which float through the air,

[00:24.03](4)missing the majority of smells

[00:25.54]which stick to surfaces.

[00:27.66]In fact, (5)though, we are extremely sensitive to smells,

[00:31.59](6)even if we do not generally realize it.

[00:34.42]Our noses are capable of (7)detecting human smells

[00:37.44]even when these are (8)diluted to far below

[00:40.06]one part in one million.

[00:42.68]Strangely, some people find that they can smell

[00:45.71]one type of flower but not another,

[00:47.72](9)whereas others are sensitive

[00:49.14]to the smells of both flowers.

[00:51.35]This may be because some people do not have

[00:53.46]the genes necessary to generate (10)particular smell receptors

[00:57.12]in the nose.

[00:58.00]These receptors are the cells which sense smells

[01:01.19]and send (11)messages to the brain.

[01:03.61]However, it has been found

[01:05.22]that even people insensitive to

[01:06.84]a certain smell (12)at first

[01:08.66]can suddenly become sensitive to it

[01:10.47]when (13)exposed to it often enough.

[01:13.30]The explanation for insensitivity to smell

[01:16.01]seems to be that the brain finds it

[01:17.93](14)inefficient to keep all smell receptors working all the time

[01:21.77]but can (15)create new receptors if necessary.

[01:24.50]This may (16)also explain

[01:25.91]why we are not usually sensitive to our own smells

[01:29.14]--we simply do not need to be.

[01:30.96]We are not (17)aware of the usual smell

[01:33.11]of our own house,

[01:34.52]but we (18)notice new smells

[01:35.93]when we visit someone else's.

[01:37.84]The brain finds it best to

[01:39.37]keep smell receptors (19)available for unfamiliar

[01:42.09]and emergency signals

[01:43.71](20)such as the smell of smoke,

[01:45.11]which might indicate the danger of fire.