Biphenyl (or diphenyl or phenylbenzene or 1,1′-biphenyl or lemonene) is an organic compound that forms colourless crystals and has a distinctively pleasant smell.
Biphenyl occurs naturally in coal tar, crude oil, and natural gas and can be isolated from these sources via distillation. It can also be synthesised by using a Grignard reagent such as phenylmagnesium bromide and reacting it with bromobenzene. Biphenyl is insoluble in water, but soluble in typical organic solvents. The biphenyl molecule consists of two connected phenyl rings. Lacking functional groups, it is fairly non-reactive. It will, however, participate in many of the reactions that are typical for benzene, for example, substitution reactions upon treatment with halogens in the presence of a Lewis acid.
Biphenyl is one of the most thermally stable of all organic compounds. It is combustible at high temperatures producing carbon dioxide and water when combustion is complete. Partial combustion produces carbon monoxide, smoke, soot, and low molecular weight hydrocarbons.
It is an aromatic hydrocarbon with a molecular formula (C6H5)2. It is notable as a starting material for the production of polychlorinated biphenyls (PCBs). Biphenyl is an intermediate for the production of a host of organic compounds such as emulsifiers, optical brighteners, crop protection products, and plastics. 
Environmental Effects 
Biphenyl dissolves poorly when mixed with water. Most releases of biphenyl to the environment are to air. In air, biphenyl breaks down to other chemicals or settles as dry deposits to water or land. Biphenyl attaches to solid material in water. Microorganisms living in water and in soil break down biphenyl to other chemicals. Because it is a solid that binds to soil, biphenyl is not likely to move through the ground and enter groundwater. Plants and animals may store small amounts of biphenyl.
Sources and Routes of Exposure [2,3]
Exposure can occur in the workplace or in the environment following releases to air, water, land, or groundwater. Biphenyl enters the body when people breathe air contaminated with biphenyl or consume food or water contaminated with biphenyl. It can also be absorbed through skin contact. It does not remain in the body due to its breakdown and removal.
Biphenyl may come from many different sources including:
- Industry sources: Releases from industries producing, using, or handling biphenyl, or where it is used as a heat transfer agent (for example, electrical transformers).
- Diffuse sources, and industry sources included in diffuse emissions data
- Sub-threshold facilities: Biphenyl enters the aquatic environment in wastewater effluents from textile mills that use it as a dye carrier; from industrial processes; and from leaking heat exchangers. In addition, biphenyl is released to the atmosphere as a fume during its use as a heat transfer fluid and, to a lesser extent, by volatilisation from soil and water.
- Natural sources: It occurs naturally in trace amounts, mostly in crude oil.
- Consumer products: Where used as a mould retardant in citrus fruit wrappers, in formation of plastics, optical brighteners, and hydraulic fluids. No consumer products should intentionally contain biphenyl. However, it may be found as a contaminant in some foods, and possibly in dyed products such as textiles.
Health Effects 
In workers, acute exposure to high levels of biphenyl has been observed to cause eye and skin irritation and toxic effects on the liver, kidneys, and central and peripheral nervous systems. Symptoms include headache, gastrointestinal pain, nausea, indigestion, numbness and aching of limbs, and general fatigue.
In humans, chronic exposure is characterised mostly by central nervous system symptoms, such as fatigue, headache, tremor, insomnia, sensory impairment, and mood changes; however, such symptoms are rare. EPA has not established a Reference Concentration (RfC) for biphenyl. The Reference Dose (RfD) for biphenyl is 0.05 milligrams per kilogram body weight per day (mg/kg/d) based on kidney damage in rats. No data on the carcinogenic effects of biphenyl in humans are available. EPA considers available studies on carcinogenicity in mice and rats to be inadequate. One study of mice orally exposed to biphenyl did not result in any increased incidence of tumours. A second study found tumours in treated and control rats, but their occurrence was not thought to be related to biphenyl administration. EPA has classified biphenyl as a Group D, not classifiable as to human carcinogenicity.
First Aid Measures
- Eye Contact: Check for and remove any contact lenses. Immediately flush eyes with running water for at least 15 minutes, keeping eyelids open. Get medical attention immediately.
- Skin Contact: In case of contact, immediately flush skin with plenty of water for at least 15 minutes while removing contaminated clothing and shoes. Cover the irritated skin with an emollient. Wash clothing before reuse. Thoroughly clean shoes before reuse. Get medical attention immediately.
- Serious Skin Contact: Wash with a disinfectant soap and cover the contaminated skin with an anti-bacterial cream. Seek medical attention.
- Inhalation: If inhaled, remove to fresh air. If not breathing, give artificial respiration. If breathing is difficult, give oxygen. Get medical attention.
- Ingestion: Do NOT induce vomiting unless directed to do so by medical personnel. Never give anything by mouth to an unconscious person. Loosen tight clothing such as a collar, tie, belt or waistband. Get medical attention if symptoms appear.
Exposure Controls & Personal Protection
Use process enclosures, local exhaust ventilation, or other engineering controls to keep airborne levels below recommended exposure limits. If user operations generate dust, fume or mist, use ventilation to keep exposure to airborne contaminants below the exposure limit.
Personal protective equipment including splash goggles, lab coat, dust respirator (be sure to use an approved/certified respirator or equivalent) and gloves should be worn when handling biphenyl.
In case of large spills, splash goggles, full suit, dust respirator, boots and gloves should be worn. A self contained breathing apparatus should be used to avoid inhalation of the product. Suggested protective clothing might not be sufficient; consult a specialist BEFORE handling this product.
Exposure Limits [3,5]
- TWA: 0.2 from ACGIH (TLV) [United States] 
- TWA: 1.3 from ACGIH (TLV) [United States] 
- Worksafe Australia: Maximum 8-hour time weighted average (TWA) exposure: 0.2 ppm.