Chemicals Better Left Untouched

In the vast world of chemistry, there are many chamicals that can ruin your day - or your whole life.

But not all chemicals are nasty in the same way: some are toxic, others corrosive. There are chemicals that can detonate more or less on a whim, and others that ignite very easily.

So it's difficult to say what is the Nastiest Substance in the World; the competition should at least be broken into categories. I won't enter into the topic of how easy or difficult it is to achieve protection from the various chemicals, or how likely exposition is, because it would be too long a discussion.

Acute toxicity: Ricin, nerve agents and other toxins are at the top of the list. These cause an array of unpleasant effects that culminate in death, which is rather rapid but not painless. Another piece of work is dimethyl sulphate: it causes blisters on skin and necrosis of eyes and respiratory tract.

Chronic toxicity: Asbestos is pretty bad, in the vicinity of aromatic amines and azoic dyes. They cause various ilnesses, among them types of cancer which are hard to treat. Anything with an aromatic ring has at least the potential to bind to DNA and screw it up; there is a pretty long list of other reactive groups that can possibly cause cancer.

Flammability: Carbon disulphide, with its autoignition temperature of 90 - 100 °C. It presents other reactive hazards and it's toxic, too. Many low molecular weight ethers are extremely flammable as well. Vapors behave as gases, which means that they can travel quite a distance before they reach a remote source of ignition.

Explosion: Organic perchlorate esters are prone to explode if the chemists in the lab have the wrong haircut, but someone embarked on a project to test the limits of divine benevolence and prepared fluorine perchlorate. The stuff is so unstable that it detonates upon crystallization. Ethers, if stored in contact with oxygen, produce unstable peroxydes which have caused many accidents - especially when the young chmistry student applies excessive force to remove a ground glass stopper from a bottle buried for years in the depts of some laboratory cabinet. The infamous explosive TATP belongs to the same category. Though not explosive in themselves, strong oxydizers will easily produce explosions and fires if they come into contact with combustible substances. Liquid oxygen is well-known for that behaviour, and treated with due respect (and those who don't show respect don't live for long). But if liquid oxygen is bad, liquid fluorine is devilish: it is an even stronger oxydizer, and corrodes everything except some fluorinated polymers and some metals, because passivation protects them. It is hypergolic with hydrocarbon fuels and many other substances. But even fluorine can be topped by...

The Nastiest of Them All: Chlorine Trifluoride. Even less domesticated than fluorine, it is extremely reactive and corrosive. It causes everything except passivated metals to ignite and burn - yes, concrete does burn in contact with ClF3, as does flesh. It was investigated as a rocket fuel, but it proved to be too reactive even for that. Its only practical use nowadays is as cleaning agent in chemical vapor deposition chambers for semiconductor production - making it the most extreme cleaning process I'm aware of.
Ah, the stuff is also higly toxic. But it does not cause cancer, hooray!