Persistent Organic Pollutants

In the article "Persistent Organic Pollutants," I noticed that β-hexachlorocyclohexane (HCH, historically known as BHC, for benzene hydrochloride) was erroneously referred to as lindane (C&EN, July 16, page 6). Actually, the γ-isomer of HCH is the classic organochlorine pesticide known as lindane.

It does merit noting that a mixture of major HCH isomers including alpha, beta, gamma, and delta was formerly marketed widely around the world as "technical-grade BHC." In the U.S. and Europe, we tended to favor use of lindane because it was the isomer that exhibited the greatest degree of pesticidal efficacy.

Interestingly, β-HCH (or β-BHC) is the most hydrophobic and least reactive of the isomers as all chlorines occupy more commodious equatorial positions. In lindane, three chlorines are equatorial and three are axial.

Daniel W. Elliott
Lawrenceville, N.J.

An interesting and important addendum to "Persistent Organic Pollutants" is that nature generates numerous POPs, most notably polybrominated diphenyl ethers and 25 newly discovered polyhalogenated bipyrroles that have no industrial counterpart. Some of these natural polybrominated diphenyl ethers are identical to metabolites of anthropogenic POPs.

Recent work by Christopher M. Reddy, Walter Vetter, and others (Science 2005, 307, 917; Environ. Pollut. 2006, 144, 336 and 2007, 145, 668; Mar. Pollut. Bull. 2006, 52, 572; Mar. Ecol. Prog. Ser. 2006, 324, 167; Chemosphere 2006, 62, 197 and 2007, 66, 2011; Arch. Environ. Contam. Toxicol. 2006, 51, 135) shows that these natural POPs, which are believed to be biosynthesized by marine bacteria and sponges, bioaccumulate in fish, birds, marine mammals, and in the milk of Eskimo women who consume whale blubber. In some cases, the levels of these natural POPs dwarf those of man-made organohalogens. For example, the polychlorinated bipyrrole "Q1," which is ubiquitous in the marine environment and can be viewed as a PCB isostere, has been measured at 14 ppm in an Australian dolphin.

The natural origin of these organohalogens is beyond doubt, as established both by Reddy's radiocarbon data and his discovery of several polybrominated diphenyl ethers, halogenated bipyrroles, and Q1—but not DDE, a DDT metabolite, or any halogenated industrial chemical—in archived whale oil from the final voyage in 1921 of the Charles W. Morgan whaling ship years before the industrial production and bioaccumulation of organobromine flame retardants and other POPs.

Although the health consequences of these natural POPs are as yet unknown, any risk assessment of POPs must recognize and evaluate nature's contribution to environmental pollution.

Gordon W. Gribble
Hanover, N.H.