Modern chemical weapons were introduced during World War I in an effort to reduce the deadlock of trench warfare. But they are subject to topography and weather patterns. As munitions have become more precise, their tactical advantage is being eroded. Today, they horrify more than they contribute to victories on the ground. Their indiscriminate nature and unpredictability, coupled with the sometimes-gruesome effects they have make them effective weapons of fear.

Below are five of the worst chemical weapons:

Most toxic: VX

VX belongs to organophosphorus compounds and is classified as a nerve agent because it affects the transmission of nerve impulses in the nervous system. It is odorless and tasteless in its pure form, and appears as a brownish oily liquid.

Developed in the UK in the early 1950s, VX is particularly potent because it’s a persistent agent: Once it’s released into the atmosphere it’s slow to evaporate. Under regular weather conditions, VX can persist for days on surfaces, while it can last for months in very cold conditions. “VX vapor is heavier than air,” which means that when released, “it will sink to low-lying areas and create a greater exposure hazard there.” Such characteristics make VX potentially useful as an area-denial weapon.

VX is also a fast-acting agent. Symptoms can appear only seconds after exposure. They include salivation, constriction of the pupils and tightness in the chest. As with other nerve agents, VX works by affecting the enzyme (acetylcholinesterase) that acts as the body’s ‘off switch’ for glands and muscles. With the enzyme blocked, molecules constantly stimulate the muscles. As the muscles spasm, they tire. Death is caused by asphyxiation or heart failure. While it is possible to recover from exposure, tiny amounts of the agent can be lethal.

Most recently used: Sarin

In September 2013, the UN confirmed that a chemical weapons attack involving specially designed rockets that spread sarin over rebel-held suburbs of the Syrian capital took place the month before. UN Secretary General, Ban Ki-Moon stated that this was the “most significant confirmed use of chemical weapons against civilians since Saddam Hussein used them in Halabja in 1988.”

Sarin (also known as GB) is a volatile but toxic nerve agent. A single drop the size of the head of a pin is enough to kill an adult human rapidly. It is a colorless and odorless liquid at room temperature, but evaporates rapidly when heated. After release, sarin will spread into the environment rapidly and present an immediate but short-lived threat. Similar to VX, “symptoms include headaches, salivation and secretion of tears, followed by gradual paralysis of the muscles” and possible death.

Sarin was developed in 1938 in Germany when scientists were researching pesticides. The Aum Shinrikyo cult used it in a 1995 attack on the Tokyo subway. While the attack caused mass panic, it only killed 13 because the agent was dispersed in liquid form. In order to maximize casualties, not only must the sarin be a gas, but the particles need to be small enough so they are easily absorbed through the lining of the lungs, yet heavy enough that they are not breathed back out. Sarin is not easy to weaponize.

The quality of the agent also matters. Sarin (and VX) is susceptible to degradation, especially if it isn’t pure. Iraq’s sarin for example, had a shelf life of about one to two years. While the degraded products are still toxic, they can’t be used as weapons. Although Syria’s CW stockpile was growing old and the agents were likely to have deteriorated significantly, the sarin used in the attack on the Ghouta suburbs on August 21, 2013 was higher quality than that used in the 1995 Tokyo attack or in Halabja. But it was still a far cry from the sarin produced by the United States and the Soviet Union.

Most popular: Mustard Gas

Also known as sulphur mustard, this agent gets its name from its trademark rotten mustard or garlic and onion odor. It belongs to the group of blister agents (or vesicants) that work by targeting the eyes, respiratory tract and skin, first as an irritant and then as poison for the body’s cells. It’s particularly grisly and slow acting. When skin is exposed to it, it reddens and burns for a few hours before large blisters appear and cause severe scarring and pain. Eyes will swell, tear and possibly go blind a few hours after exposure, and when inhaled or ingested, victims will experience sneezing, hoarseness, coughing up blood, abdominal pain and vomiting.

But exposure to mustard gas is not always fatal. When it was first used in WW1, it killed only 5 percent of those exposed. It became a popular chemical weapon—used in both world wars, during the civil war in Yemen and the Iran-Iraq war—because of its properties.

Along with its gruesome physical effects, mustard gas is chemically stable and very persistent. Its fumes are more than six times heavier than air and stay near the ground for several hours. This made it particularly useful for filling and contaminating enemy trenches. It remains toxic for a day or two under average weather conditions and from weeks to months under very cold conditions. What’s more, persistency can be increased by “thickening” the agent: dissolving it in nonvolatile solvents. It poses significant problems for protection, decontamination and treatment.

Mustard gas is relatively easy to produce, with readily available early precursors. It also retains its quality for a long time. For example, German munitions used in the world wars are still periodically dug up in Belgium and the agents are barely degraded.

Mustard gas forces enemy troops to wear full protective equipment thereby degrading efficiency. But the protective gear doesn’t always work. Gas masks, for example, are often not enough. During the Iran-Iraq war, mustard gas seeped through the masks as young Iranians’ beards (grown for religious purposes) broke the seal of the mask. Mustard gas also easily penetrates through clothes, shoes or other materials.

Most dangerous: Phosgene

To this day, phosgene is considered one of the most dangerous existing chemical weapons. It was first used in combination with chlorine gas on December 19, 1915, when Germany dropped 88 tons of the gas on British troops, causing 120 deaths and 1069 casualties. During World War I, it accounted for 80 percent of all chemical fatalities. Although it is not as toxic as sarin or VX, it’s much easier to make, which makes it more accessible to all.

Phosgene is an industrial chemical used in the fabrication of plastics and pesticides. It’s made by exposing chlorinated hydrocarbon compounds to high temperatures. In other words, it can be made at home by exposing chloroform to UV-light for a few days.

Phosgene is a choking agent that works by attacking lung tissue. Initial likely symptoms of coughing, choking, tightness in the chest, nausea, and occasionally vomiting occur minutes after exposure. This may seem quick but it actually means that victims continue to inhale it until symptoms become apparent. Delayed effects can occur up to 48 hours after exposure.

At room temperature and pressure, it’s an almost colorless gas that smells of freshly cut grass in low concentrations. It’s nonflammable and evaporates when heated above eight degrees, which makes it volatile. But its vapor density is more than three times that of air, which means that it’ll linger in low-lying areas, including trenches.

Most attainable: Chlorine

In June, OPCW inspectors declared that “pulmonary irritating agents such as chlorine, have been used in a systematic manner in a number of attacks” in Syria after Assad pledged to give up its chemical weapons. This led to doubts about the value of the U.S.-Russian deal on the elimination of Syria’s chemical weapons.

Chlorine is a readily available industrial chemical with many peaceful uses, including as bleach in paper and cloth, to make pesticides, rubber, and solvents and to kill bacteria in drinking water and swimming pools. It’s the perfect example of a problematic dual-use chemical. Chlorine did not figure in Assad’s initial stockpile declaration in October and was not removed with the rest of Syria’s chemical weapons last month. Despite its dual-use nature, chlorine’s use as a chemical weapon is still banned under the Chemical Weapons Convention (CWC).

Chlorine gas is yellow-green colored and has a strong smell similar to bleach. Like phosgene, it is a choking agent, which obstructs breathing and damages tissues in the body. It can easily be pressurized and cooled to liquid state so that it can be shipped and stored. Chlorine spreads quickly and stays close to the ground because it is heavier than air. Though it is less lethal than other chemical agents, chlorine is dangerous because it’s easy to manufacture and disguise.

Special mention: Riot control agents

While not strictly banned by the CWC (it doesn’t apply to domestic law enforcement), riot control agents have often been used with devastating effects. Their temporary effects include crying, uncontrollable blinking, burning in the throat, sneezing, coughing, vomiting, and at times temporary blindness.