Hexachlorocyclopentadiene (HCCPD), also known as C-56, is an organochlorine compound that is a precursor to several pesticides. This colourless liquid is an inexpensive reactive diene. Many of its derivatives proved to be highly controversial, as studies showed them to be persistent organic pollutants. Collectively, the pesticides derived from hexachlorocyclopentadiene are called the cyclodienes. 
Hexachlorocyclopentadiene is used as a raw material in manufacturing other chemicals including certain pesticides. Most of the HCCPD in the environment results from releases during its production and disposal. It is also used to make flame-retardants, resins that won’t burn, shockproof plastics, esters, ketones, fluorocarbons, and dyes. HCCPD has no end uses of its own.
In the Environment 
HCCPD can be released to the air as a vapour during its production and use. However, it does not remain in the air very long since it is usually broken down to other substances by sunlight and by reaction with other chemicals in the air. Half of the HCCPD released to the air is removed in less than one day. When mixed with water at room temperature, only 2.1 milligrams will dissolve in a litre of water (2 parts per million or 2 ppm). In a stream or small river, the HCCPD near the surface will evaporate to the air. Sunlight on the water will cause HCCPD to change quickly into other chemicals. About half of the HCCPD in the water will be changed to other chemicals by the light in only four minutes. The HCCPD that gets into soil binds to decaying plant and animal matter. If the soil is sandy, the HCCPD can move through the soil and reach the water that is under the ground. When soil that contains HCCPD also contains solvents like gasoline, paint thinners, and acetone, these liquids will help carry the HCCPD through the soil to lakes, rivers, or wells. Bacteria can change HCCPD in the soil to other chemicals, but scientists do not know the nature of these compounds. About half of the HCCPD in the soil will be changed to other chemicals by bacteria in 1–2 weeks. HCCPD has been known to build up in fish, but only in very small amounts. It is unknown whether HCCPD accumulates in plants, milk, or animals used for food.
Sources & Routes of Exposure
Sources of Exposure 
- If you live near a hazardous waste site where HCCPD or HCCPD-derived pesticides were disposed, you might be exposed to HCCPD in the air. HCCPD has not been reported in outdoor air in city, suburban, and rural areas. In most areas, the concentration of HCCPD in the air should be low because this chemical is not widely used.
- HCCPD is not commonly found in surface water or drinking water, so exposure by this route is unlikely. However, it may be formed during chlorination of water containing humic acid.
- HCCPD may be present in soils that have recently been treated with the pesticides, endosulfan or pentac, because it is sometimes found as an impurity in these pesticides. The soils near a landfill where these pesticides (including those no longer used, such as aldrin, chlordane, dieldrin, endrin, heptachlor, and isodrin) or waste HCCPD were disposed might also contain HCCPD, but, since it binds to organic matter in soils, it is less likely to be free to affect you.
- It is highly unlikely that you will be exposed to HCCPD in the foods you eat, although you could be exposed to very small amounts if you catch and eat fish that lived in HCCPD-contaminated water.
- The highest exposures to HCCPD are likely to occur in people who are involved in the production or use of HCCPD, who handle pesticides made from it, or who treat wastes that contain it. These people can be exposed by breathing air contaminated with HCCPD, or by skin and eye contact with the vapours or liquid.
Routes of Exposure 
HCCPD can enter the body via the following routes:
- skin absorption
- skin and/or eye contact
Health Effects 
Hexachlorocyclopentadiene is very toxic to humans. It is a severe eye, skin, and pulmonary irritant in humans. Inhalation of the chemical causes tearing, sneezing, and salivation, and skin contact can cause blisters and burns. The major target organ for acute hexachlorocyclopentadiene toxicity is the lung, with cough, chest pains, and difficulty in breathing reported in humans. Nervousness, headaches, and abdominal cramps are other symptoms reported from hexachlorocyclopentadiene toxicity. Tests involving acute exposure of rats have shown hexachlorocyclopentadiene to have extreme toxicity by inhalation exposure, moderate toxicity by oral exposure, and high to extreme toxicity by dermal exposure.
Epidemiologic studies on workers have not shown any significant differences in mortality between workers exposed to hexachlorocyclopentadiene and those in the general population. However, these studies are limited by short follow-up periods, lack of data on cigarette smoking, and other factors. Chronic exposure to hexachlorocyclopentadiene, via inhalation, has been studied in animals, with effects noted in the lung, liver, kidney, and blood. EPA has established a Reference Concentration (RfC) of 0.0002 milligrams per cubic metre (mg/m3) for hexachlorocyclopentadiene, based on respiratory effects in rats. The Reference Dose (RfD) for hexachlorocyclopentadiene is 0.006 milligrams per kilogram body weight per day (mg/kg/d) based on stomach lesions in rats.
No information is available regarding the reproductive or developmental effects of hexachlorocyclopentadiene in humans. Animal studies have not reported birth defects from exposure to hexachlorocyclopentadiene by gavage (placing the chemical experimentally in the stomach), and no information is available regarding reproductive or developmental effects from inhalation exposure.
Epidemiologic studies have not demonstrated any differences in mortality between hexachlorocyclopentadiene-exposed workers and the general population. The observed mortality included deaths from cancer, as well as from other diseases. The National Toxicology Program (NTP) completed a 2-year inhalation study and concluded that there was no evidence of carcinogenic activity in rats and mice. EPA has classified hexachlorocyclopentadiene as a Group D; not classifiable as to human carcinogenicity.
First Aid Measures
- Eyes: Immediately flush eyes with plenty of water for at least 15 minutes, occasionally lifting the upper and lower eyelids. Get medical aid immediately.
- Skin: Get medical aid immediately. Immediately flush skin with plenty of water for at least 15 minutes while removing contaminated clothing and shoes.
- Ingestion: Do not induce vomiting. Get medical aid immediately. Call a poison control centre.
- Inhalation: Remove from exposure and move to fresh air immediately. If breathing is difficult, give oxygen. SPEED IS ESSENTIAL, OBTAIN MEDICAL AID IMMEDIATELY. Do not use mouth-to-mouth resuscitation if victim ingested or inhaled the substance; induce artificial respiration with the aid of a pocket mask equipped with a one-way valve or other proper respiratory medical device.
Fire Fighting Measure
- As in any fire, wear a self-contained breathing apparatus in pressure-demand, MSHA/NIOSH (approved or equivalent), and full protective gear.
- Substance may react with water, and may release corrosive and/or toxic gases.
- Do NOT use water directly on fire.
- Use foam, dry chemical, or carbon dioxide.
Exposure Controls and Personal Protection
- Facilities storing or utilising HCCPD should be equipped with an eyewash facility and a safety shower.
- Use only under a chemical fume hood.
Personal Protective Equipment
- Eyes: Wear appropriate protective eyeglasses or chemical safety goggles as described by OSHA’s eye and face protection regulations in 29 CFR 1910.133 or European Standard EN166.
- Skin: Wear appropriate protective gloves to prevent skin exposure.
- Clothing: Wear appropriate protective clothing to prevent skin exposure.
- Respirators: A respiratory protection program that meets OSHA’s 29 CFR 1910.134 and ANSI Z88.2 requirements or European Standard EN 149 must be followed whenever workplace conditions warrant respirator use.
OSHA: The Occupational Safety & Health Administration has established limits on exposure to HCCPD in order to protect workers exposed on the job. The limit is 0.01ppm in the air for an 8-hour workday over a 40- hour workweek.
- NIOSH: The National Institute for Occupational Safety and Health (NIOSH) suggests the same limit for workplace air.
- EPA has recommended guidelines on how much HCCPD can be present in drinking water. The maximum contaminant levels (MCL) and maximum concentration level goal (MCLG) for drinking water are 50 ppb.
- EPA recommends that exposures in children should not exceed 2 ppm in water for 10-day periods or no more than 0.7 ppb for up to 7 years.
- If adults are exposed for more than 7 years, the EPA recommends that exposure levels should not exceed 50 ppb.
- HCCPD has been named a hazardous substance by EPA. If quantities equal to or greater than one pound are released to the environment, the National Response Centre for the federal government must be told immediately.
Safe Work Australia: Safe Work Australia has established a time weighted average concentration for HCCPD of 0.1ppm and 0.11 mg/m3 over an 8-hour workday.