Dichlorobenzene (mixed isomers)

Para-dichlorobenzene (p-DCB) is by far the most commercially important isomer — it is the active ingredient in moth balls, toilet bowl deodorizers, and air fresheners [1]. Ortho-dichlorobenzene (o-DCB) is an industrial solvent used in chemical synthesis and as a carrier solvent for grain fumigation. Meta-dichlorobenzene is the least produced [2]. Para-DCB was introduced as a 'safer' replacement for naphthalene moth balls in the mid-20th century and became widely used in households — but its potential carcinogenicity became apparent from animal studies in the 1980s showing liver and kidney tumors [1]. The pervasive use of p-DCB as a household chemical means that virtually all Americans have detectable serum p-DCB levels, with the National Exposure Report showing 95th percentile values well above the 50th percentile reference — driven primarily by moth ball and deodorizer use [2].

The most significant exposure pathway for most people is inhalation of p-DCB from moth balls and toilet deodorizer blocks used in the home — these products slowly sublimate, raising indoor air concentrations significantly [1]. People who use moth balls heavily and store them in closets may have indoor p-DCB levels exceeding OSHA permissible exposure levels [2]. Occupational inhalation occurs in the o-DCB solvent and p-DCB moth ball manufacturing industries. Environmental contamination from industrial sites using chlorobenzenes as solvents is found at Superfund sites [1].

Para-DCB is metabolized by CYP2B6 and CYP3A4 to reactive epoxide intermediates and catechol metabolites that cause oxidative stress and covalent binding to cellular proteins [1]. Chronic exposure induced liver carcinomas and kidney tumors (particularly in male rats through an alpha-2u-globulin nephropathy mechanism) in rodent studies; EPA classifies p-DCB as a Group C (possible) carcinogen [2]. Ortho-DCB causes liver and kidney toxicity at occupational levels. Acute high-level exposure to DCB isomers causes CNS depression, hepatotoxicity, and hemolytic anemia [1].

Heavy household users of p-DCB moth balls and toilet deodorizers — especially people with small, poorly ventilated closets [1]. Workers in DCB manufacturing and specialty chemical operations [2]. Private well users near Superfund sites with chlorobenzene contamination [1].

1. Replace moth balls with alternatives (cedar blocks, lavender sachets) — moth balls are a near-unnecessary source of carcinogen exposure [1]. 2. Stop using p-DCB toilet deodorizer blocks — switch to fragrance-free cleaning alternatives [2]. 3. If you use moth balls, store items in sealed bags outside the home (garage) and air them out thoroughly before bringing back inside [1]. 4. Test your well if near industrial Superfund sites with chlorinated solvent contamination [2].