Smoke screen
The titanium tetrachloride smoke is an irritant and unpleasant to breathe.
It is dispensed from aircraft to create vertical smoke curtains, and during World War II it was a favorite smoke generation agent on warships.
Goggles and a respirator should be worn when in contact with the smoke, full protective clothing should be worn when handling liquid FM. In direct contact with skin or eyes, liquid FM causes acid burns.

Phosphorus: Red phosphorus and white phosphorus (WP) are red or waxy yellow or white substances. White phosphorus is pyrophoric - can be handled safely when under water, but in contact with air it spontaneously ignites. It is used as an incendiary. Both types of phosphorus are used for smoke generation, mostly in artillery shells, bombs, and grenades. 
White phosphorus smoke is typically very hot and may cause burns on contact. Red phosphorus is less reactive, does not ignite spontaneously, and its smoke does not cause thermal burns - for this reason it is safer to handle, but cannot be used so easily as an incendiary.
Aerosol of burning phosphorus particles is an effective obscurant against thermal imaging systems. However, this effect is short-lived. After the phosphorus particles fully burn, the smoke reverts from emission to absorption. While very effective in the visible spectrum, cool phosphorus smoke has only low absorption and scattering in infrared wavelengths. Additives in the smoke that involve this part of the spectrum may be visible to thermal imagers or IR viewers.

Dyes: Various signalling purposes require the use of colored smoke. The smoke created is a fine mist of dye particles, generated by burning a mixture of one or more dyes with a low-temperature pyrotechnic composition, usually based on potassium chlorate and lactose (also known as milk sugar).
Colored smoke screen is also possible by adding a colored dye into the fog oil mixture. Typical white smoke screen uses titanium dioxide (or other white pigment), but other colors are possible by replacing titanium dioxide with another pigment. When the hot fog oil condenses on contact with air, the pigment particles are suspended along with the oil vapor. Early smoke screen experiments attempted the use of colored pigment, but found that titanium dioxide was the most light scattering particle known and therefore best for use in obscuring troops and naval vessels. Colored smoke became primarily used for signaling rather than obscuring. In today's  military, smoke grenades are found to be non-cancer causing, unlike the 1950s AN-M8 model.

Sulfonic acid: The smoke generator on the Medium Mark B tank used sulfonic acid.

Tactics: 

History: The first documented use of a smoke screen was circa 2000 B.C. in the wars of ancient India, where incendiary devices and toxic fumes caused people to fall asleep.
It was later recorded by a Greek historian, Thucydides, who described that the smoke created by the burning of sulphur, wood and pitch was carried by the wind into Plataea (428 B.C.) and later at Delium (423 B.C.) and that at Delium, defenders were driven from the city walls.
In 1622, a smoke screen was used at the Battle of Macau by the Dutch. A barrel of damp gunpowder was fired into the wind so that the Dutch could land under the cover of smoke.
Later, between 1790 and 1810, Thomas Cochrane, 10th Earl of Dundonald (1775–1860), a Scottish Naval commander and officer in the Royal Navy who fought during the French Revolutionary and Napoleonic Wars, devised a smoke screen created through the burning of sulphur which would be used in warfare after learning about the same methods used at Delium and Plataea.
Thomas Cochrane, 10th Earl of Dundonald's grandson, Douglas Cochrane, 12th Earl of Dundonald, described in his autobiography how he spoke to Winston Churchill (who once galloped for him when he had a brigade at manœuvres in England) of the importance of using smoke-screens on the battleground, it would in turn be used in both WWI & WW2.

Land warfare: Smoke screens are usually used by infantry to conceal their movement in areas of enemy fire. They can also be used by armoured fighting vehicles, such as tanks, to conceal a withdrawal. They have regularly been used since earliest times to disorient or drive off attackers.
During the First World War the Germans used a lot of smoke screens (Nebel) to hide Batterie Pommern.
A toxic variant of the smokescreen was used and devised by Frank Arthur Brock who used it during the Zeebrugge Raid on 23 April 1918, the British Royal Navy's attempt to neutralize the key Belgian port of Bruges-Zeebrugge.
For the crossing of the Dnieper river in October 1943, the Red Army laid a smoke screen 30 kilometres (19 mi) long. At the Anzio beachhead in 1944, US Chemical Corps troops maintained a 25 km (16 mi) "light haze" smokescreen around the harbour throughout daylight hours, for two months. The density of this screen was adjusted to be sufficient to prevent observation by German forward observers in the surrounding hills, yet not inhibit port operations.
In the Vietnam War, "Smoke Ships" were introduced as part of a new Air Mobile Concept to protect crew and man on the ground from small arms fire. In 1964 and 1965, the "Smoke Ship" was first employed by the 145th Combat Aviation Battalion using the UH-1B.

Naval warfare: There are a number of early examples of using incendiary weapons at sea, such as Greek fire, stinkpots, fire ships, and incendiaries on the decks of turtle ships, which also had the effect of creating smoke. The naval smoke screen is often said to have been proposed by Sir Thomas Cochrane in 1812, although Cochrane's proposal was as much an asphyxiant as an obscurant. It is not until the early twentieth century that there is clear evidence of deliberate use of large scale naval smokescreens as a major tactic.
During the American Civil War, the first smoke screen was used by the R.E. Lee, running the blockade and escaping the USS Iroquois.
The use of smoke screens was common in the naval battles of World War I and World War II.

See also: Early thermal weapons
Military camouflage
PT boat
Smoke bomb

References: 

External links:  The dictionary definition of smokescreen at Wiktionary
 Media related to Smoke screens at Wikimedia Commons