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As per the security rules, i think upto 100ml of liquid ( medicines, perfumes etc) can be taken on-board by a passenger but is it possible that, that 100ml can be converted to fumes/gas and contaminate the air inside the aircraft?

Any such incident has ever been reported?

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    $\begingroup$ 100 mL of myriad substances could easily stink up the entire plane, among other things. A few notorious chemicals from biology labs spring to mind: TEMED (an amine that smells like rotten fish) and beta-mercaptoethanol (a thiol that smells like rotten eggs), and neither of those are particularly special. $\endgroup$ – Nick T Jun 15 '15 at 18:39
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    $\begingroup$ Surströmming is more than capable of stinking up a whole airliner. This is the reason why it is forbidden on some airlines. And you can eat it too. $\endgroup$ – f470071 Jun 17 '15 at 8:11
  • $\begingroup$ The durian fruit (found mostly in SE Asia) can easily stink up any modern airliner. It is banned in most hotels due to its strong smell. So not sure why the hate on an a can of air. $\endgroup$ – Burhan Khalid Aug 13 '18 at 16:58
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To answer your first question: it depends what you mean by "contaminate". The air in an aircraft cabin is constantly being renewed every two or three minutes (source (Airbus) - Boeing aircraft are similar), and the airflow pattern is such that the air only flows "across" the cabin, not along it. In other words, any contaminant you introduce will only reach the passengers seated in the same row as you (and perhaps the rows directly behind and in front of you), and will only stay there for a few minutes before the air exchange carries it overboard.

About 50% of the air which leaves the cabin is recycled back into the cabin, but it passes through filters first, which will stop any particles but will let gases through. Statistically, the concentration of whatever was introduced will drop to nearly zero after two passes through the ventilation system (due to dilution with fresh outside air).

My understanding is that the "100ml" rule is more designed to stop passengers from building liquid explosives aboard an aircraft (source (TSA)) than to stop them introducing harmful contaminants into the air.

I don't know of any incidents where an airborne substance was used maliciously in an airplane cabin, but you would get a similar effect through an on-board fire. The aircraft's ventilation system is designed to clear the cabin and flight deck of smoke within a short period of time (often by disabling the air recirculation system and increasing the fresh air flow into the aircraft).

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    $\begingroup$ "Statistically, the concentration of whatever was introduced will drop to nearly zero after two passes through the ventilation system" Huh? If each pass recycles 50% of the air, then two passes will still leave 25% of the contamination. That's not even close to zero. $\endgroup$ – David Richerby Aug 13 '18 at 13:47
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    $\begingroup$ ...assuming the air is fully contaminated, of course. $\endgroup$ – Burhan Khalid Aug 13 '18 at 17:00
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Gases

For it to be converted to a gas, it would have to either be stored in a pressurized container or be a liquid that happens to vaporize at room temperature in the relatively small pressure difference between the surface and the cabin pressure altitude. Pressurized gas containers are not allowed on board U.S. airline flights unless they are:

  1. Medical oxygen cylinders with in-tact and unmodified regulator valves,
  2. Part of a life vest, or
  3. Verifiably empty.

From the TSA Prohibited Items List:

Allowed in checked or carry-on luggage:

Small compressed gas cartridges (Up to 2 in life vests and 2 spares. The spares must accompany the life vests and presented as one unit)

Not allowed in any luggage:

Fire extinguishers and other compressed gas cylinders

From the "Can I bring my...?" search blank on the same page with the search term "compressed gas cylinder":

Except for personal medical oxygen cylinders, you can only carry an EMPTY compressed gas cylinder on board a plane. To be permitted (in either carry-on or checked baggage), it must be clearly visible to the Transportation Security Officer (TSO) that the cylinder is empty. Personal oxygen is permitted if the regulatory valve has not been tampered with or removed.


Aerosols

Aerosols are allowed, but are subject to the same 3-1-1 rule as other liquids, that is, they must be in a 100 mL or less container which is then stored inside a 1 L / 1 qt. zip-top bag. As Lightsider mentioned, these will almost certainly be filtered out of the air by the air filters in the ventilation system, as they are not in gaseous form (and likely are relatively large particles and/or molecules.)

From the same TSA page using the search term "aerosols," you get the following:

You may carry liquids, gels and aerosols in your carry-on bags only if they adhere to the 3-1-1 rule: containers must be 3.4 ounces or less; stored in a 1 quart/liter zip-top bag; 1 zip-top bag per person, placed in the screening bin. Larger amounts of non-medicinal liquids, gels, and aerosols must be placed in checked baggage.

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You don't even need to take a separate liquid. Here is one example of a flight turning around due to a horrible smell. https://www.vice.com/en_us/article/bnpevw/one-year-on-still-thinking-about-poo-plane-303. Here is another example, in which the man who was the source of the smell (necrotic tissue from an infection that happened while he was on vacation) later died. http://www.foxnews.com/travel/2018/06/27/unbearable-smelling-passenger-that-caused-plane-s-emergency-landing-dies-from-tissue-necrosis.html. There is also a SpiceJet incident similar to the first (British Airways) one.

But if one were intent on taking a liquid onboard to contaminate the air, yes, it would be possible, and there are certainly many extremely foul smelling or even toxic liquids that would evaporate if spread broadly enough. Fortunately, the toxic compounds are difficult to obtain.

The air volume of a 747 is roughly 1,000 cubic meters. (Other planes are much smaller, so easier to contaminate.) When 100 ml of a liquid vaporizes, its volume depends on its density, temperature, pressure, and molecular weight. The vaporized volumes of 100 ml of trimethyl amine, cadaverine, putrescine, or various sulfides would be within an order of magnitude of 10 liters, and 10 liters is 1 part in 1 million of 1,000 cubic meters. Any of those would likely be foul smelling enough to require turning the plane around. (It might be easier to just bring some rotting fish, though.)

Cabin air filters and the ventilation system would gradually reduce the concentration somewhat, but if they were truly effective, those example flights wouldn't have been cut short. Ten passes through the system at 50% dilution per pass would reduce the original concentration by a factor of approximately 1,000, but that will take a while, and there are many compounds that stink even at 1 part in a billion. (And once dozens of passengers have vomited, that will add butyric acid smell to the mix.)

If anyone figures out you brought the stench onboard, you probably risk being beaten and certainly prosecuted. So yes, it's probably possible, but so far seems never to have occurred.

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If you have the right substances, it's possible.

In theory you'd only need 50 grams of Sarin to reach LD50 in a 747. Poor dispersion will mean much less than a 50% casualty rate, but 100 ml is enough to cause some damage even just spilled - it's volatile and evaporates on its own.

As the Aum Shinrikyo attack has shown, it's possible to produce privately. But a well-ventilated aircraft is probably not as good a place for chemical attacks as a crowded theater. Takes more time to get aid, though.

The 100ml rule is meant to protect against binary explosives, as well as innocent but dangerous liquids such as gasoline. Yes, an educated person apparently really decided it was worth bringing on a flight. Hull loss.

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