Thu, 15 Jan 2004 11:25:05 -0800 in article
<33qd00hpcevktbr47fcrjq9d3018e2rpct@4ax.com> Pete <peteh@coomes.invalid>
wrote:

Fish oil contains negligible amounts of mercury. As for other contaminant, if
you want to believe commercial hype, you should by concentrated pharmaceutical
grade fish oil, which has gone through both molecular and fractional
distillation processes.  I found nothing about these distillation processes in
the context of fish oil from Medline, but for whatever it's worth, many
commercial sites are keen to boast about these processes.

Medline references:

Foran SE, Flood JG, Lewandrowski KB.
Measurement of mercury levels in concentrated over-the-counter fish oil
preparations: is fish oil healthier than fish?
Arch Pathol Lab Med. 2003 Dec; 127(12): 1603-5.
<URL:http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1
4632570&dopt=Abstract>

   "...  The fish oil brands examined in this manuscript have negligible
   amounts of mercury and may provide a safer alternative to fish
   consumption. ..."

Koller H, Luley C, Klein B, Baum H, Biesalski HK.
[Contaminating substances in 22 over-the-counter fish oil and cod liver oil
preparations: cholesterol, heavy metals and vitamin A]
Z Ernahrungswiss. 1989 Mar;28(1):76-83. German.
PMID: 2718527 [PubMed - indexed for MEDLINE]
<http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2
718527&dopt=Abstract>

   "Fish oil capsules are increasingly used by self-medicating
   patients. We studied 22 commercial fish oil and menhaden oil
   preparations in respect to accompanying substances that could be
   harmful. The substances measured were: cholesterol as determined by
   gas liquid chromatography, heavy metals measured by atomic
   absorption, and vitamin A as determined by high-performance liquid
   chromatography (HPLC). The contents of cholesterol and heavy metals
   were in ranges which can be regarded as negligible; the content of
   vitamin A in menhaden oils, however, was found in amounts which
   warrant that pregnant women do not exceed the dosage as recommended
   by the manufacturers."

van de Ven WS.
Mercury and selenium in cod-liver oil.
Clin Toxicol. 1978;12(5):579-81.
PMID: 679643 [PubMed - indexed for MEDLINE]
<http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=6
79643&dopt=Abstract>

   "Cod-liver oil preparations were analyzed for mercury and selenium.
   The conclusion can be drawn that the intake of mercury and selenium
   in the normal use of these preparations will be very low."

Jacobs MN, Covaci A, Schepens P.
Investigation of selected persistent organic pollutants in farmed Atlantic
salmon (Salmo salar), salmon aquaculture feed, and fish oil components of the
feed.
Environ Sci Technol. 2002 Jul 1; 36(13): 2797-805
<URL:http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1
2144249&dopt=Abstract>

Hilbert G, Lillemark L, Balchen S, Hojskov CS.
Reduction of organochlorine contaminants from fish oil during refining.
Chemosphere. 1998 Sep; 37(7): 1241-52.  
<URL:http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9
734319&dopt=Abstract>

Jimenez B, Wright C, Kelly M, Startin JR.
Levels of PCDDs, PCDFs and non-ortho PCBs in dietary supplement fish oil
obtained in Spain.
Chemosphere. 1996 Feb; 32(3): 461-7.
<URL:http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8
907223&dopt=Abstract>

Commercial sites:

Impurities in fish oil
<URL:http://www.iherb.com/mercury.html>

   "... Molecular distillation is the only method (currently) that can remove
    metals, pcbs and other toxins to below detectable levels for human
    consumption. ..."

<URL:http://news.drsears.com/archives/july/omega3.htm>

   "... Some manufacturers have relied on "molecular
  distillation" as a touch-up process to remove a small
  proportion of these contaminants. Although this is an
  improved "clean-up" step, molecular distillation does not
  remove all the deadly contaminants that they claim and
  that the consumer is led to believe. Molecularly
  distilled fish oil products do not qualify as
  "pharmaceutical grade" regardless of what the
  manufacturer says. In fact, it is the crude starting
  material for the production of OmegaRx. Additional and
  costly processing is then required to guarantee that
  contaminants and saturated fats have been substantially
  removed from the oil. This can only be done by a complex
  engineering process known as "fractional distillation,"
  the only guarantee that you are consuming
  "pharmaceutical-grade" oils. If a fish oil product does
  not undergo fractional distillation, then it still
  contains relatively high levels of toxins. ..."

--
Matti Narkia