Tear gas is again filling the streets of Ferguson as police try to clear violent protests in the wake of a grand jury's decision not to bring charges against the officer who shot Michael Brown.
The unarmed black teenager was killed by Darren Wilson, who is white, on 9 August, sparking weeks of unrest including vandalism and looting.
Scenes on Monday night, when police cars were burned, buildings set alight and shots fired, were worse according to the chief of police.
From demonstrations in Ferguson to the Israeli-Palestinian conflict, the recent use of tear gas to quickly disperse crowds of people has caused controversy, particularly as it is banned in warfare but allowed to control civilian crowds.
Police in the town, in the US state of Missouri, have used tear gas, stun grenades and smoke bombs to disperse thousands of demonstrators over three months of protests over Mr Brown's death.
Similarly, Israeli soldiers fired tear gas and stun grenades to disperse pro-Palestinian protesters in the West Bank in mid-July.
Photographs from both scenes, despite being over 6,500 miles apart, show the similar acrid clouds of smoke filling streets as people try to flee.
In fact, those protesting against the situation in the Gaza Strip were so concerned by the situation in Ferguson that they took to social media to share tips on how to deal with the attacks.
So, what is tear gas and why are its effects so feared?
What is tear gas?
The most commonly used tear gas contains the chemical agent 2- chlorobenzaldene malononitrile (CS).
Despite its name, tear gas is not a gas, but an aerosol. CS is solid at room temperature and mixed with liquid or gas dispersal agents when used as a weapon designed to activate pain-sensing nerves.
Early forms of the tear gas were first used in World War I, both by France and Germany.
But CS gas was developed in 1928, when American chemists Ben Corson and Roger Stoughton synthesised its active component.
Tear gas use across the world
Tear gas use across the world
13 August, 2014: An Al Jazeera television crew, covering demonstrators protesting the shooting death of teenager Michael Brown, scramble for cover as police fire tear gas into their reporting position on in Ferguson, Missouri.
Scott Olson/Getty Images
17 August, 2014: A woman has her face doused with milk after suffering the effects of tear gas used by police at a protest of the death of Michael Brown in Ferguson, Missouri.
8 August, 2014: Israeli border guards fire tear gas grenades towards Palestinian protesters following a demonstration in support of Gaza after Friday prayers at the Hawara checkpoint, east of the West Bank city of Nablus.
MUSA AL-SHAER/AFP/Getty Images
28 July, 2014: A Palestinian protester holding his national flag stands amid smoke after tear gas was fired by Israeli security forces during clashes in the West Bank village of Bilin, following a demonstration in support of Palestinians from the Gaza Strip.
ABBAS MOMANI/AFP/Getty Images
12 June, 2014: A demonstrator kicks a tear gas grenade during an anti-World Cup protest, on the morning the Brazilian mega-city hosts the tournament's opening match, in Sao Paulo on. Police fired tear gas, stun grenades and rubber bullets to break up dozens of protesters gathering near a Sao Paulo.
Miguel Schincariol/AFP/Getty Images
3 February, 2012: A man sprayed a milky liquid in the eyes of a protester suffering from the effects of tear gas inhalation during clashes between protesters and riot police near the interior ministry February 3, 2012 in Cairo, Egypt. The protest followed the deaths of 74 football fans who were killed in clashes between rival fans following the match between al-Masry and al-Alhy in Port Said, Egypt.
Carsten Koall/Getty Images
15 October, 2011: A protester throws back a teargas to Policemen during a demonstration in Rome on Protesters set fire to a government building, torched cars and smashed bank windows in Rome in the worst violence of worldwide demonstrations against corporate greed and government cutbacks.
FILIPPO MONTEFORTE/AFP/Getty Images
June 10, 2003: A teacher suffers of heavy teargas used by police to disperse crowd after a demonstration against pension reform plan at the Concorde square in Paris.
Pascal Le Segretain/Getty Images
9/12 South Korea
10 April 1994: Reformist Buddhist monks, masked in case of tear gas, form a skirmish line in Seoul to protect other monks as they scale ladders in an attempt to occupy the secretariat building of the Chogye Temple Some 2,000 riot police later dislodged the reformists who were trying to oust a senior monk they accuse of corruption.
CHOO YOUN-KONG/AFP/Getty Images
21 December 1987: Israeli riot policemen throw teargas at Palestinian rioters in north Jerusalem, declared as 'Peace day', in support of Palestinians in the occupied territories. Hundreds of Palestinian protesters demonstrated in support of victims shot dead during demonstrations in the occupied territories since the beginning of December 1987.
MENAHEM KAHANA/AFP/Getty Images
10th August 1942: Demonstrators lying on the ground to avoid clouds of tear gas released by the police when they refused to disperse after protesting against the arrest of Gandhi and other National Congress leaders.
5th August 1939: Tear gas being used against striking automobile workers in Cleveland, Ohio.
Fox Photos/Getty Images
What are the laws surrounding its use?
According to the Chemical Weapons Convention of 1993, which was signed by almost every nation in the world, tear gas is banned in warfare but legal in domestic riot control.
Video: Ferguson protesters teargassed
“Tear gas under the Geneva Convention is characterized as a chemical warfare agent, and so it is precluded for use in warfare, but it is used very frequently against civilians,” Sven-Eric Jordt, a nerve gas expert at Yale University School of Medicine, told National Geographic. “That’s very illogical.”
What are its effects?
Tear gas works by irritating the mucous membranes of the eyes, nose, mouth, and lungs.
Typically, the effects kick in after around 30 seconds, and include a burning, watery sensation in the eyes, difficulty breathing, chest pain, excessive saliva and skin irritation. But those who face heavy exposure can also suffer from vomiting and diarrhoea.
After the person has escaped into fresh air, the effects should subside after around 10 minutes, Neil Gibson, an analyst with IHS Jane's, an intelligence and security publication, told BBC News.
Why are people pouring milk on their faces?
People have been pictured pouring milk or milky substances on their faces after suffering from the effects of tear gas, which is just one of the home-made remedies people have claimed will help with symptoms.
Woman getting treated with McDonald's milk after getting hit with tear gas (via Getty) pic.twitter.com/8TLBZzi7jy— Matt Novak (@paleofuture) August 18, 2014
In Ferguson, protesters who have been exposed to tear gas have been pictured having milk poured over their faces in McDonald's, reportedly by the fast-food chain's employees.
The activist tweeting from the Israel/Gaza conflict told Americans in Ferguson not to pour water on their faces, but to use milk or coke instead to stem its effects.
It is thought that milk soothes the pain induced by lachrymatory agents, or tear gas. In an online flier circulated by people affiliated with the Occupy movement in 2011, antacids such as Maalox dissolved in water was advised as a method of relief from the effects of tear gas, sprayed onto people's eyes and mouths, and to be swallowed.