What is Hexachlorobenzene?

Hexachlorobenzene (HCB) is a fully chlorinated aromatic hydrocarbon with the molecular formula C₆Cl₆. It is a white crystalline solid, insoluble in water, but very soluble in fats, oils, and organic solvents. HCB is one of the most persistent environmental pollutants, capable of bioaccumulating in the food chain and biomagnifying in animals and humans. Though it is no longer intentionally manufactured in the United States and is banned under the Stockholm Convention on Persistent Organic Pollutants, HCB continues to be produced as a by-product or impurity during the manufacture of several chlorinated solvents and pesticides (e.g., pentachloronitrobenzene, chlorothalonil, and lindane).

What is Hexachlorobenzene used for?

HCB has no current commercial uses. It was previously used as a fungicide on agricultural seeds such as wheat, sorghum, and onions. HCB was also formerly used in the manufacture of fireworks, ammunition, rubber, aluminium, dyes, and wood preservatives. Today, HCB is found primarily as an unintended by-product in the production of chlorinated industrial chemicals, solvents, and in waste emissions from facilities such as chloralkali plants and municipal waste incinerators.

In the Environment

HCB is highly persistent in the environment and breaks down very slowly. It does not dissolve well in water and instead binds strongly to soil and sediment. Most HCB found in aquatic environments remains attached to particles at the bottom of lakes and rivers. Due to its fat solubility, HCB accumulates in the tissues of animals, especially fish, marine mammals, and birds, and is known to concentrate up the food chain. It can also accumulate in plants such as grasses, vegetables, and grains.

Sources & Routes of Exposure

Sources of Exposure

• Inhalation near industrial or waste sites producing or disposing of HCB.
• Occupational exposure via inhalation or skin contact in industries where HCB is formed as a by-product.
• Ingestion of contaminated food (e.g., dairy, meat, fish).
• Drinking contaminated water.
• Skin contact with contaminated soil, especially near waste disposal sites.
• Breastfeeding infants may be exposed via contaminated breast milk.

HCB is listed as a pollutant of concern by the EPA Great Waters Programme due to its persistence, bioaccumulation potential, and toxicity.

Routes of Exposure

• Inhalation: A less significant route for the general population, but possible near contaminated sites.
• Oral ingestion: The primary exposure route, mostly from contaminated food.
• Dermal contact: Especially important for individuals living near contaminated soil or waste sites, and particularly relevant for children.

Health Effects

Acute Effects

• There is no available information on acute (short-term) health effects of hexachlorobenzene in humans.
• Animal studies in rats and mice indicate that hexachlorobenzene has low-to-moderate acute toxicity when taken orally.

Chronic Effects

A 4-year poisoning incident involving contaminated bread revealed that humans who ingested hexachlorobenzene developed a liver disorder (porphyria cutanea tarda) with associated skin lesions. Very little data is available on the effects of inhalation exposure in humans or animals. Chronic oral exposure in animals has shown impacts on the liver, kidneys, skin, immune system, and blood.

• EPA has not established a Reference Concentration (RfC) due to insufficient data.
• CalEPA has set a chronic inhalation reference exposure level of 0.003 mg/m³.
• The Reference Dose (RfD) is 0.0008 mg/kg/day, based on liver effects in rats.

Reproductive/Developmental Effects

• In the same human poisoning incident, abnormal physical development was reported in young children.
• Animal studies show that HCB can cross the placenta, accumulate in foetal tissues, and cause neurological, teratogenic, liver, and immune system effects in offspring.
• Decreased survival rates of newborns were also observed in exposed animals.

Cancer Risk

• There is inadequate human data on the cancer-causing potential of hexachlorobenzene. However, oral studies in animals link HCB exposure to tumours in the liver, thyroid, and kidneys.
• EPA classifies hexachlorobenzene as a Group B2 probable human carcinogen.
• The inhalation unit risk is estimated at 4.6 × 10⁻⁴ (µg/m³)⁻¹.

Safety

First Aid Measures

• Eyes: Flush eyes with water for at least 15 minutes, lifting both upper and lower eyelids occasionally. Seek medical attention.
• Skin: Remove contaminated clothing and shoes. Rinse affected skin with plenty of water for at least 15 minutes. Wash clothing before reuse. Seek medical attention.
• Ingestion: If the person is conscious and alert, give 2–4 cupfuls of milk or water. Do not give anything by mouth to an unconscious person. Seek medical attention.
• Inhalation: Immediately remove the person from exposure and move to fresh air. If breathing has stopped, give artificial respiration. If breathing is difficult, administer oxygen. Seek medical attention.

Exposure Control & Personal Protection

Engineering Controls

• Use adequate general or local exhaust ventilation to keep airborne concentrations of hexachlorobenzene below permissible exposure limits.

Personal Protective Equipment (PPE)

• Eyes: Wear chemical safety goggles or protective eyewear compliant with OSHA (29 CFR 1910.133) or EN166.
• Skin: Use appropriate protective gloves to prevent skin contact.
• Clothing: Wear protective clothing to avoid skin exposure.
• Respiratory Protection: If exposure limits are exceeded or symptoms occur, use a NIOSH/MSHA- or EN 149-approved respirator. Follow OSHA 29 CFR 1910.134 or applicable standards for respiratory protection.

Regulation

United States

• The EPA has recommended that drinking water should not contain more than 0.05 milligrams of hexachlorobenzene per litre of water (0.05 mg/L) in water that children drink, and should not contain more than 0.2 mg/L in water that adults drink for longer periods (about 7 years). The EPA has set a maximum contaminant level of 0.001 mg/L in drinking water.
• The EPA requires that spills or accidental releases into the environment of 10pounds or more of hexachlorobenzene be reported to the EPA.
• ACGIH: The American Conference of Governmental Industrial Hygienists has set a time weighted average threshold limit value (TLV) of 0.002 mg/m3; Skin; Appendix A3 - Confirmed Animal Carcinogen with Unknown Relevance to Humans

Australia

• No national guidelines.