Carcinogenicity and Toxicity of Aromatic Hydrocarbons

Benzene and the larger fused-ring aromatic compounds called polycyclic aromatic hydrocarbons (PAHs) -- naphthalene, benzo[a]pyrene and their relatives, which are produced whenever an

organic fuel (wood, tobacco, coal, petrol, or even grilled meat) is burned incompletely -- are not

chemically inert in the body the way their stability toward laboratory reagents might suggest.

Both benzene itself and PAHs are recognised health hazards, and several are classified as

carcinogens (cancer-causing agents).

Metabolic activation. Aromatic hydrocarbons are, in general, too non-polar and chemically

unreactive to damage biological molecules directly. The danger instead arises because the liver's

own detoxification machinery -- cytochrome P450 enzymes, which normally oxidise foreign, fat-soluble

compounds to make them more water-soluble and easier to excrete -- oxidises the aromatic ring to

form a reactive epoxide intermediate (an oxygen bridging two adjacent ring carbons, straining

and destabilising that part of the ring). Rather than being harmlessly excreted, this epoxide is

itself chemically reactive and electrophilic; it can react covalently with the nucleophilic sites

on DNA bases (most classically, the exocyclic amino group of guanine), forming a permanent

DNA adduct. This is metabolic activation turning a relatively inert parent compound into a

genuinely dangerous, DNA-damaging species -- the body's own attempt to detoxify the chemical is

what generates the harmful intermediate.

Consequences. A DNA adduct that is not correctly repaired by the cell's DNA-repair machinery

can cause a mutation when the DNA is next replicated, and an accumulation of such mutations in

genes that control cell growth and division is a recognised route to cancer -- benzo[a]pyrene

(found in cigarette smoke, grilled/charred food, and vehicle exhaust) is one of the

best-characterised chemical carcinogens for exactly this reason, strongly linked to lung and skin

cancers. Benzene itself carries an additional, distinct toxicity: chronic occupational exposure

(historically a serious hazard in industries using benzene as a solvent) is strongly associated …