Which one of the following was used as a chemical weapon in the First World War?

1. Basics of Carbon-based Gases: CO and CO₂ (basic)

Welcome to your first step in understanding the intersection of chemistry and human health. To understand how environmental agents cause disease, we must start with the most fundamental molecules we interact with: carbon-based gases. Carbon is a unique element because it has a valency of four, meaning it can bond with four other atoms. This versatility, combined with its ability to link into long chains (a property called catenation), allows it to form a vast array of compounds Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.62. In our atmosphere, the two most significant carbon-oxygen gases are Carbon Dioxide (CO₂) and Carbon Monoxide (CO).

While both are colorless and odorless, they are produced under different conditions and affect the human body in distinct ways. Carbon Dioxide (CO₂) is a product of complete combustion (burning with plenty of oxygen). It is a natural part of the air, but in high concentrations — common in urban areas due to vehicle emissions — it acts as a pollutant that causes coughing, nausea, and bronchial irritation Geography of India, Majid Husain (McGrawHill 9th ed.), Contemporary Issues, p.38. A simple diagnostic test for CO₂ is passing it through lime water, which turns milky in its presence Science, Class VIII (NCERT 2025 ed.), Nature of Matter, p.119.

In contrast, Carbon Monoxide (CO) is formed during incomplete combustion (burning when oxygen is restricted). From a health perspective, CO is far more dangerous than CO₂. It is often called the 'silent killer' because it binds to the hemoglobin in our red blood cells much more strongly than oxygen does. This prevents the blood from carrying oxygen to vital organs, leading to rapid oxygen deprivation. These compounds are typically non-conductors of electricity because their bonding does not give rise to ions, making them stable but chemically potent in biological systems Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59.

Feature	Carbon Dioxide (CO₂)	Carbon Monoxide (CO)
Combustion	Complete (excess oxygen)	Incomplete (limited oxygen)
Detection	Turns lime water milky	Requires electronic sensors
Health Effect	Respiratory irritant/Asphyxiant	Systemic toxin (blocks oxygen)

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.62; Geography of India, Majid Husain (McGrawHill 9th ed.), Contemporary Issues, p.38; Science, Class VIII (NCERT 2025 ed.), Nature of Matter, p.119; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59

2. Industrial Gases: Water Gas and Producer Gas (basic)

In the world of industrial chemistry and energy, Water Gas and Producer Gas are two vital fuel gases derived from coal. While modern infrastructure in India has shifted significantly toward natural gas—transported through massive arteries like the 1,700 km Hazira-Vijaipur-Jagdishpur (HVJ) pipeline NCERT, Contemporary India II, p.115—the production of gas from coal remains a cornerstone of heavy industry. These gases are produced by reacting steam or air with red-hot coke or coal, transforming a solid fuel into a more manageable gaseous form that can be used for heating or as a chemical feedstock GC Leong, Certificate Physical and Human Geography, p.281.

The primary distinction between these two lies in their chemical composition and energy content. Water Gas is a mixture of Carbon Monoxide (CO) and Hydrogen (H₂). It is created by passing steam over incandescent coke. Because both components are flammable, it has a high calorific value. In contrast, Producer Gas is a mixture of Carbon Monoxide (CO) and Nitrogen (N₂), formed by passing air over hot coal. Since nitrogen is inert and does not burn, producer gas has a lower heating value but is much cheaper to manufacture.

From a human health perspective, both gases are extremely hazardous because of their high Carbon Monoxide (CO) content. CO is often called the 'silent killer' because it is colorless and odorless. When inhaled, it binds to hemoglobin in the blood 200 times more strongly than oxygen does, forming carboxyhemoglobin. This prevents the blood from carrying oxygen to vital organs, leading to rapid asphyxiation and death. This is why industrial environments using these gases require strict monitoring and ventilation.

Feature	Water Gas	Producer Gas
Primary Components	CO + H₂	CO + N₂
Manufacturing Agent	Steam passed over hot coke	Air passed over hot coke
Calorific Value	Higher (more combustible)	Lower (contains inert Nitrogen)
Industrial Use	Synthesis of methanol, fuel	Industrial heating, furnace fuel

Sources: NCERT, Contemporary India II, Manufacturing Industries, p.115; GC Leong, Certificate Physical and Human Geography, Manufacturing Industry, p.281

3. Respiratory Toxins and Human Health (intermediate)

To master the study of human health, we must understand how our respiratory system—the body's primary interface with the external atmosphere—is vulnerable to chemical agents. **Respiratory toxins** are substances that, when inhaled, cause localized damage to the lungs or utilize the pulmonary system as a gateway to cause systemic harm. These toxins are generally categorized by their physiological impact: **irritants** (which cause inflammation), **asphyxiants** (which deprive the body of oxygen), and **vesicants** (which cause blistering and chemical burns).

Many common pollutants act as potent irritants. For instance, Nitrogen Oxides (NOₓ) and Sulfur Oxides (SOₓ), largely emitted from vehicles and thermal power plants, cause intense irritation of the eyes and throat. More critically, they impair the respiratory system's ability to exchange gases by causing "inflammation of the lungs" and "impairing enzyme functions," which often leads to chronic conditions like bronchitis and asthma. Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.40. While these are often chronic issues, industrial accidents can lead to acute, lethal exposures. The 1984 leak of Methyl Isocyanate (MIC) in Bhopal remains one of history's deadliest examples, where the toxin caused immediate "internal hemorrhage" and total "lung failure." A Brief History of Modern India, Spectrum, After Nehru..., p.721.

It is also important to recognize that respiratory toxins are not exclusively man-made. Natural processes can release dangerous gases; for example, rotting seaweed is a significant source of Hydrogen Sulfide (H₂S). This highly toxic gas is not just a respiratory irritant but can cause systemic poisoning leading to "vomiting and diarrhoea." Environment, Shankar IAS Academy, Marine Organisms, p.210. Understanding the source and the specific biological pathway of these toxins is essential for both public health management and historical analysis of chemical warfare and industrial safety.

Toxin Type	Common Examples	Primary Health Impact
Irritants	NOₓ, SO₂, HCl	Inflammation, breathlessness, and bronchial asthma.
Systemic Toxins	Methyl Isocyanate (MIC)	Internal hemorrhage and rapid lung failure.
Natural Toxins	Hydrogen Sulfide (H₂S)	Systemic poisoning and gastrointestinal distress.

Sources: Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.40; A Brief History of Modern India, Spectrum, After Nehru..., p.721; Environment, Shankar IAS Academy, Marine Organisms, p.210

4. International Law: Chemical Weapons Convention (CWC) (intermediate)

The Chemical Weapons Convention (CWC) is a landmark international arms control treaty that serves as a global safeguard against some of the most horrific agents of warfare. Unlike earlier agreements that merely banned the use of such weapons, the CWC, which opened for signature in 1993 and entered into force in 1997, prohibits the entire lifecycle of chemical weapons: their development, production, acquisition, stockpiling, transfer, and use. It is uniquely powerful because it is the first disarmament treaty to provide for the verifiable elimination of an entire category of weapons of mass destruction (WMD) under international oversight.

To ensure compliance, the convention established the Organisation for the Prohibition of Chemical Weapons (OPCW), headquartered in The Hague. The OPCW conducts rigorous inspections of industrial and military sites worldwide to ensure that chemicals are used only for peaceful purposes. From a health perspective, these chemicals—ranging from blistering agents like sulfur mustard (C₄H₈Cl₂S) to nerve agents like Sarin—cause catastrophic physiological damage, often leading to long-term chronic respiratory illness or agonizing death. The CWC aims to ensure these substances never enter the human environment again.

India’s commitment to this regime is absolute. India was one of the original signatories and has fully destroyed its declared chemical weapon stockpiles under OPCW supervision. Interestingly, while India maintains a strict 'No First Use' policy regarding nuclear weapons, its national doctrine specifies a caveat: in the event of a major chemical or biological attack against Indian forces or territory, India retains the option of retaliating with nuclear weapons Indian Polity, M. Laxmikanth (7th ed.), Foreign Policy, p.611. This highlights just how seriously the international community and India view the threat of chemical warfare as a violation of humanitarian norms.

Sources: Indian Polity, M. Laxmikanth (7th ed.), Foreign Policy, p.611

5. Global Watchdogs: The OPCW (intermediate)

The Organisation for the Prohibition of Chemical Weapons (OPCW) serves as the global watchdog dedicated to the permanent elimination of chemical weapons. Established to implement the Chemical Weapons Convention (CWC) which entered into force in 1997, the OPCW ensures that chemistry is used only for peace, progress, and prosperity. While traditional security often relies on a balance of power, the CWC represents a move toward disarmament—the total surrender of a specific class of weaponry Contemporary World Politics, Textbook in political science for Class XII (NCERT 2025 ed.), Security in the Contemporary World, p.69. Currently, 193 states have acceded to the CWC, making it one of the most successful international disarmament treaties in history. From a human health perspective, the OPCW’s work is vital because chemical agents are designed to cause mass harm, disease, and agonizing death. For example, mustard gas (sulfur mustard, C₄H₈Cl₂S), infamous for its use in World War I, is a powerful vesicant (blistering agent). It doesn't just kill; it causes chronic respiratory failure, severe skin burns, and permanent blindness, often leaving victims with life-long health complications. By verifying the destruction of over 70,000 metric tonnes of declared chemical stockpiles, the OPCW directly prevents the recurrence of such public health catastrophes. Unlike many international bodies that may face financial or jurisdictional hurdles Indian Polity, M. Laxmikanth(7th ed.), National Human Rights Commission, p.476, the OPCW possesses a unique inspection regime. It has the authority to conduct 'challenge inspections' in any member state where foul play is suspected. This technical rigour earned the OPCW the Nobel Peace Prize in 2013. Its mission bridges the gap between international law and clinical toxicology, ensuring that the 'invisible killers' of the past century do not threaten human health in the 21st.

Sources: Contemporary World Politics, Textbook in political science for Class XII (NCERT 2025 ed.), Security in the Contemporary World, p.69; Indian Polity, M. Laxmikanth(7th ed.), National Human Rights Commission, p.476

6. Industrial Disasters: Bhopal Gas Tragedy (exam-level)

The Bhopal Gas Tragedy, which occurred on the night of December 3, 1984, remains the world's worst industrial disaster. It was triggered by the leak of Methyl Isocyanate (MIC) — a highly toxic chemical used in pesticide production — from the Union Carbide India Limited (UCIL) plant. The disaster was not merely a technical failure but a systemic one, involving the storage of hazardous chemicals in a densely populated urban area and a disregard for established safety protocols Rajiv Ahir, A Brief History of Modern India, After Nehru, p.721-722. From a health perspective, the gas acted as a silent killer, causing internal haemorrhage, acute lung failure, and severe ocular damage, leaving survivors with lifelong respiratory and neurological impairments.

Beyond the immediate human cost, the tragedy fundamentally altered India's legal and environmental architecture. Before 1984, environmental protection was largely handled through administrative decisions by the Planning Commission and lacked a unified legislative backbone Environment, Shankar IAS Academy, Environmental Impact Assessment, p.128. The outcry following the disaster forced the government to transition from 'advice' to 'authority,' leading to the enactment of the Environment (Protection) Act, 1986 (EPA). This 'umbrella legislation' allowed the government to coordinate activities of various central and state authorities and introduced the mandatory Environmental Impact Assessment (EIA) for industrial projects.

Feature	Pre-Bhopal (1984)	Post-Bhopal (1984)
Legal Basis	Fragmented (Wildlife Act 1972, Water Act 1974)	Unified under Environment (Protection) Act 1986
Authority	Primarily administrative decisions	Legislative power to set standards and penalties
Hazardous Waste	Minimal regulation on handling/storage	Strict Rules (1989) for generation and import

Sources: A Brief History of Modern India (SPECTRUM), After Nehru..., p.721-722; Environment (Shankar IAS Academy), Environmental Impact Assessment, p.128; Environment and Ecology (Majid Hussain), Major Crops and Cropping Patterns in India, p.88

7. Classification of Chemical Warfare Agents (exam-level)

Chemical warfare agents (CWAs) are toxic chemical substances used to cause intentional death or harm. Unlike biological agents which are living pathogens, CWAs are non-living chemical compounds. They are primarily classified based on their physiological effects—that is, how they specifically attack the human body. This classification is vital for medical response and international regulation under bodies like the Organisation for the Prohibition of Chemical Weapons (OPCW).

We generally divide these agents into four major functional categories:

• Blister Agents (Vesicants): These cause severe skin, eye, and mucosal pain, leading to large, fluid-filled blisters. Sulfur Mustard (Mustard Gas) is the most prominent example. It is notorious for its persistence in the environment and its ability to cause chronic respiratory damage and long-term health issues Environment, Shankar IAS Academy (ed 10th), Environment Issues and Health Effects, p.439.
• Choking Agents (Pulmonary Agents): These attack the respiratory system. Chemicals like Phosgene (COCl₂) and Chlorine cause the lungs to fill with fluid (pulmonary edema), making it impossible for the victim to breathe.
• Nerve Agents: These are among the most lethal. They disrupt the electrical impulses and chemical reactions at nerve endings Science, Class X (NCERT 2025 ed.), Control and Coordination, p.101. By inhibiting the enzyme acetylcholinesterase, they cause muscles to become overstimulated, leading to paralysis and respiratory failure. Examples include Sarin and VX.
• Blood Agents: These prevent the body's cells from using oxygen. Hydrogen Cyanide is a classic example; it acts rapidly by interfering with cellular respiration, effectively causing asphyxiation at the microscopic level.

Many of these substances, or their precursors, are also found in industrial settings. For instance, compounds like chloromethyl ethers or acetic acid used in manufacturing can cause similar upper and lower airway irritation if exposure is high Environment, Shankar IAS Academy (ed 10th), Environment Issues and Health Effects, p.438-439.

Category	Mechanism of Action	Classic Example
Vesicants	Destroys skin/tissue on contact; causes blisters	Mustard Gas
Nerve Agents	Interrupts nervous system signals to muscles	Sarin
Choking Agents	Damages lung tissue; causes fluid buildup	Phosgene
Blood Agents	Stops cells from utilizing oxygen	Hydrogen Cyanide

Sources: Environment, Shankar IAS Academy (ed 10th), Environment Issues and Health Effects, p.438-439; Science, Class X (NCERT 2025 ed.), Control and Coordination, p.101

8. Mustard Gas: Properties and Historical Use (exam-level)

Mustard Gas, or sulfur mustard (C₄H₈Cl₂S), is perhaps the most infamous chemical warfare agent in history. Despite its name, it is not actually a gas at room temperature; it is a viscous, oily liquid that is typically dispersed as a fine mist or aerosol. It gets its name from its characteristic smell, which resembles mustard, garlic, or horseradish. In chemistry, sulfur is a non-metal that often forms compounds with distinct physical properties, such as being insoluble in water Science-Class VII, The World of Metals and Non-metals, p.53. Similarly, mustard gas is poorly soluble in water but highly soluble in oils and fats, which allows it to penetrate human skin and clothing with terrifying ease.

The primary danger of mustard gas lies in its classification as a vesicant (blister-forming agent). When it comes into contact with the skin, eyes, or lungs, it acts as a powerful alkylating agent, meaning it chemically attaches to and damages the DNA within cells. This damage leads to cell death or mutations. A unique and psychological aspect of mustard gas is its delayed effect; victims often do not feel any immediate pain upon exposure. Symptoms, such as severe skin blistering, temporary blindness, and respiratory distress, typically appear 2 to 24 hours later. As we know from laboratory safety, even common chemicals like caustic soda can cause severe skin burns Science-Class VII, Life Processes in Plants, p.143, but mustard gas is far more insidious due to this delayed onset and its ability to cause deep, systemic injury.

Historically, mustard gas was first used effectively on a large scale during World War I by the German army in July 1917 at the Battle of Ypres (hence its other name, Yperite). While other gases like chlorine and phosgene were more immediately lethal, mustard gas was strategically superior because of its persistence. Unlike gases that dissipate quickly with the wind, this oily substance could remain active in the soil, on equipment, and in trenches for days or even weeks. This forced soldiers to wear heavy protective gear for long periods and made entire battlefields uninhabitable. This persistence is a hallmark of sulfur-based compounds, which can be chemically stable and difficult to neutralize Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.128.

Sources: Science-Class VII, The World of Metals and Non-metals, p.53; Science-Class VII, Life Processes in Plants, p.143; Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.128

9. Solving the Original PYQ (exam-level)

Having just explored the properties of lethal gases and their physiological impacts, this question tests your ability to apply applied chemistry to a specific historical timeline. In the UPSC Prelims, science is rarely tested in the abstract; it is frequently linked to major global events or environmental legacies. You have learned that chemical agents are classified by their primary effects—such as vesicants (blistering agents), asphyxiants, and blood agents. This question specifically targets the most iconic vesicant used to break the brutal stalemate of trench warfare during the early 20th century.

When analyzing the options, your reasoning should focus on battlefield persistence and delivery. While many chemicals are toxic in a laboratory, a successful weapon must remain effective in an open, windy field. Mustard gas (sulfur mustard) was known as the "King of the Battle Gases" because it was an oily liquid that evaporated slowly, meaning it settled into trenches and remained active for days, causing debilitating blisters on the skin and lungs. Therefore, (C) Mustard gas is the correct choice, famously used at scale by July 1917, a fact corroborated by historical records in the IARC Monographs on the Evaluation of Carcinogenic Risks to Humans.

To avoid common UPSC traps, you must distinguish between toxicity and tactical utility. Carbon monoxide and Hydrogen cyanide are indeed lethal, but they are "light" gases that disperse too rapidly in open air to be effective in trench warfare. Water gas is a classic distractor; while it sounds like a chemical agent, it is actually an industrial fuel consisting of carbon monoxide and hydrogen. UPSC often includes these industrial terms to see if you can differentiate between general chemical compounds and specific chemical warfare agents.