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I was told by my chemistry teacher that $\ce{HCN}$ smells like almonds. She then went on to tell a story about how some of her students tried to play a prank on her by pouring almond extract down the drain to make her think that they had inadvertently created $\ce{HCN}$ gas. She said that she knew that it wasn't $\ce{HCN}$ because if she had smelled the almond scent, then she would have already been dead.

I never asked her, but how do people know $\ce{HCN}$ smells like almonds if they would die before they knew what it smells like?

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16  
That's an exaggeration. Some compounds would probably kill you before you have the chance to feel their smell, but $\ce{HCN}$ would not. – Ivan Neretin Mar 1 at 5:46
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I have a compound in my synthesis right now that smells like almonds. I instinctively am weary of the smell, despite having never (knowingly) smelled HCN. In addition to what everyone else has said, it should be noted that bitter almond oil (apparently) actually contains HCN. I knew amygdalin was a component, but I found out a while back that, supposedly, there is free HCN in the oil (likely a decomposition product of amygdalin). See "side effects" section here. – SendersReagent Mar 1 at 7:56
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As to the comment by your teacher, the smell of the almond extract was probably quite pronounced, so perhaps her comment was along the lines of, "that much HCN would have produced symptoms already, so it isn't HCN". – DevSolar Mar 1 at 10:42
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@DGS. Weary? Wary? Wavy? Tired? – TRiG Mar 1 at 22:30
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The thing I said before I said "That was a joke." – SendersReagent Mar 3 at 20:11
up vote 63 down vote accepted

The odour threshold for HCN is in fact quite a bit lower than the lethal toxicity threshold. Data for hydrogen cyanide can be found in many places, but here and here are a couple of good references. That subset of the human population that can detect bitter almonds do so at a threshold of 0.58 to 5ppm. The lethal exposure dose is upwards of 135ppm. That's a whole 100ppm range in which to detect and report the fragrant properties.

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12  
You did beat me by the length of a shower and a shave :D Further data on inhalation effects can be found in the SCOEL Recommendation 115. One should however keep in mind that a number of people can't smell HCN at all. – Klaus Warzecha Mar 1 at 6:50
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I wonder who was the first to volunteer for this experiment. "100ppm and you won't die, I swear, just sniff this jar and tell me what you smell!" – hownowbrowncow Mar 1 at 15:39
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One way to look it at this is that our olfactory system wouldn't have been much of an evolutionary advantage if it didn't warn us about poisons before they killed us. – Crashworks Mar 2 at 0:47
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@Crashworks True! But also why we can make use of sentinel species like the canary in the coalmine :-) – long Mar 2 at 0:59
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Missing a major point: inhaling 100 ppm for a short time has absolutely no ill effects, as you need to accumulate a certain amount (about 10 mg) of cyanide in your blood for it to kill you. One breath would have to be highly concentrated to kill you - on the order of 10,000 ppm. See my answer for details. – Floris Mar 2 at 1:41

Recognize that a whiff of most toxins, even in high concentration, will probably not kill you. You need a sufficient concentration in your blood - which means you have to actually get a certain number of HCN molecules to penetrate across the mucosa of the lung and into the blood stream. Typical breathing volume is about 500 mL (tidal volume), about 1/40th of a mole, so a single normal breath of 100 ppm HCN would contain 2.5 µmol of HCN.

According to the CDC, humans can tolerate inhalation of 50 ppm HCN for half an hour "without immediate or delayed effects", while 100 ppm for more than half an hour may be fatal.

The Mayo clinic reports that blood cyanide levels over 2 µg / ml are toxic; if you have 5 liters of blood, that corresponds to 10 mg. The molar mass of HCN is 27, meaning that 10 mg is 370 µmol. At 2.5 µmol / breath, you should be able to take 148 breaths; at 6 breaths per minute, that would be about 25 minutes.

All these numbers are quite consistent, and it tells you that you can tolerate a whiff of cyanide without ill effects. But it's a good idea, once you smell it, to open the windows, turn on the extractor fans, and get yourself to some fresh air.

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Gatterman reports (Org. Synth. 1927, 7, 50, as a footnote) that people who smoke regularly have enhanced sensitivity to the smell of cyanide gas, and he recommend smoking while preparing it!

Organic Synthesis Collective Volume 1 1941 314-315

Just opening the NaCN container, most regular (and former regular) smokers can smell the trace amount of HCN formed from the water vapor in the air.

I suspect your teacher was exaggerating slightly.

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Even if cyanide gas isn't a reactive fuel for a cigarette butt, surely other gases in the lab are -- that doesn't sound like sound advice to me :P – cat Mar 2 at 0:17
6  
It isn't sound advice, the recommendation is from 1927. Old-fashioned, outdated and dangerous. I wasn't smoking when I was making hydrogen cyanide. – Lighthart Mar 2 at 15:28
    
It may not be sound advice, but it is true that smokers smell cyanide at much lower levels. It is probably one of the few positive benefits of smoking. – matt_black Mar 3 at 17:12

Hydrogen cyanide $(\ce{HCN})$ is variously described as smelling of bitter almonds, marzipan, ratafia, or peach kernels. While some people can smell $\ce{HCN}$ at very low concentrations, many people cannot perceive the odour at all. The odour threshold is about $1{-}6\ \mathrm{mg/m^3}$ for people who are actually sensitive to the odour of $\ce{HCN}$.

Inhalation of $\ce{HCN}$ at low concentrations above the odour threshold is not necessarily lethal. Toxicity of inhaled cyanides is strongly dependent upon concentration and exposure time. By way of comparison, guideline figures taken from Marrs, T. C.; Maynard, R. L.; Sidell, F. R. Chemical warfare agents: toxicology and treatment; John Wiley & Sons, 1996; p 204 are shown in the following table:

$$\textbf{Inhalation toxicity to humans}\\ \begin{array}{lll} \hline \text{Time} & \mathrm{LC_{50}} & \mathrm{LCt_{50}} \\ \text{in}\ \mathrm{min} & \text{in}\ \mathrm{mg\ m^{-3}} & \text{in}\ \mathrm{mg\ m^{-3}\ min} \\ \hline \hphantom{0}0.25 & 2\,400 & \hphantom{0\,}660 \\ \hphantom{0}1 & 1\,000 & 1\,000 \\ 10 & \hphantom{0\,}200 & 2\,000 \\ 15 & \hphantom{0\,}133 & 4\,000 \\ \hline \end{array}$$

Hence, for example, it is possible to smell $\ce{HCN}$ for a period of $1\ \mathrm{min}$ at a concentration of $100\ \mathrm{mg/m^3}$, which is well above the odour threshold $(1{-}6\ \mathrm{mg/m^3})$ but well below the lethal concentration for this period $(1\,000\ \mathrm{mg/m^3})$. However, after an extended period, inhalation of $\ce{HCN}$ at this concentration will probably become lethal.

Even at acute exposure to high concentrations of $\ce{HCN}$, it is possible to smell $\ce{HCN}$ before the toxic effects occur. This may be illustrated by a description given in Vedder, E. B. The Medical Aspects of Chemical Warfare; Williams and Wilkins, 1925; p 187:

In an atmosphere containing a lethal concentration an odour of bitter almonds is noticed. This is followed by a sensation of constriction of the throat, giddiness, confusion and indistinct sight. The head feels as though the temples were gripped in a vice, and there may be pain in the back of the neck, pain in the chest, with palpitation and laboured respiration. Unconsciousness occurs and the man drops. From this moment if the subject remains in the atmosphere of hydrocyanic acid for more than two or three minutes death almost always ensues, after a brief period of convulsions followed by failure of respiration.

However, the figures given above are extremely uncertain. Nevertheless, note that $\ce{HCN}$ does not obey Haber’s rule $(c \cdot t = k)$. One important reason for the dependency of the toxicity upon concentration is the existence of various pathways for detoxication. Detoxication explains the ability to withstand very low concentrations of cyanide indefinetely. However, it is unlikely that detoxication plays any significant role in acute cyanide poisoning.

A significant contribution to the uncertainty of acute inhalation toxicity is caused by the variable breathing rate. Standard reference values taken from ICRP 66 (1994) are shown in the following table:

$$\textbf{Breathing rates for adult males}\\ \begin{array}{lll} \hline \text{Acitivity} & \text{Breathing rate} \\ & \text{in}\ \mathrm{m^3\ h^{-1}} \\ \hline \text{Resting (sleeping)} & 0.45 \\ \text{Sitting awake} & 0.54 \\ \text{Light exercise} & 1.5 \\ \text{Heavy exercise} & 3.0 \\ \hline \end{array}$$

However, such values may not be applicable to acute cyanide poisoning because of the respiratory stimulation caused by $\ce{HCN}$.

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The frightening thing about that description from Vedder is that it sounds like he is describing an experimental observation: "the man drops"; with the "death almost always ensues" phrase suggesting an N>1 experiment. Chemical warfare indeed. – Floris May 19 at 10:40

Some gases are toxic and cumulatively desensitizing to smell, like Hydrogen Sulphide. It used to be added (a while ago) to gas supplies to alert of gas leaks, because, heck, you think there's no longer a gas leak and you strike a match...

This was stopped when people were poisoned by it (without the associated exploding fireballs). Your nose no longer smells it after a few minutes and a toxic concentration can be inhaled without the usual warning signs.

Current smell additives to gas supplies are non-desensitizing.

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Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. For more information in general have a look at the help center. At the moment this reads more like a comment than an actual answer - could you elaborate a little more. With a bit more rep, you will be able to post comments on any question/answer. While this is interesting and correct, it does unfortunately not answer the question. – Martin - マーチン Mar 3 at 14:36

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