Introduction
This topic covers the occupational intoxications caused by benzene and three related aromatic hydrocarbons widely used in industry: xylene, styrene and toluene. For the exam you should be able to explain their sources and absorption, the key differences in toxicity, the acute and chronic clinical picture, the laboratory and biological monitoring, the principles of treatment, and how such cases are handled in medical expertise.
Etiology and exposure sources
All four compounds are aromatic hydrocarbons with a benzene ring, obtained mainly from petroleum and coal tar. They are volatile and lipophilic, so the main route of exposure at work is inhalation of vapours, with additional absorption through intact skin for liquids. Typical risk occupations include petroleum refining, coke and chemical production, rubber and synthetic glue manufacture, paint, varnish and printing work, furniture and shoe manufacture, dry cleaning, and work with fuels. Tobacco smoke and car exhaust are important background sources of benzene.
Pathogenesis
Because of their lipophilicity, these compounds rapidly cross cell membranes and the blood-brain barrier, which explains the prominent acute central effects. Most of the absorbed dose is distributed to fatty tissue and then slowly released.
- Narcotic effect: the solvents disturb neuronal membranes, causing CNS depression, which is the main acute mechanism for all four compounds.
- Irritant effect: direct contact damages the mucous membranes of the eyes and airways and the skin, which leads to dermatitis with repeated exposure.
- Metabolic activation (mainly benzene): cytochrome P450 enzymes, chiefly CYP2E1, convert benzene to reactive metabolites such as benzene oxide, hydroquinone and catechol. These cause oxidative stress, DNA and chromosome damage and inhibition of bone marrow progenitor cells, producing myelotoxicity.
- Leukaemogenicity: chronic benzene exposure damages haematopoietic stem cells and increases the risk of acute myeloid leukaemia and myelodysplastic syndrome. Toluene and xylene are not established leukaemogens in the same way.
- Neurotoxicity: chronic exposure to toluene and styrene affects central and peripheral nerve function, and styrene and toluene also damage the visual and vestibular-auditory pathways.