Chuck,

I thought I'd add a little detail to the origins of the Prop 65 300 ppm
number for lead in cables.  This number came from rudimentary exposure
estimates trying to link the Prop 65 Maximum Allowable Dose Level (MADL)
for reproductive toxicity for lead (which is 0.5 microgram/day) to a
consumer's lead exposure resulting from use of a lead-containing PVC cable
on an electronic device.  Basically, Mateel & associates ran course lab
wipers over PVC cables containing up to 2% lead, measured how many
micrograms of Pb came off on the wiper, and then took a swag at estimating
how much of that lead might be ingested by a typical consumer on a typical
day.  Adding in safety factors, they came up with a 'safe' concentration
limit of 300 ppm for lead in the PVC cables. At this level, it was
determined that the ingested exposure of a typical consumer would not
exceed the Prop 65 MADL of 0.5 microgram/day for Pb.  This work was done
in early 2001, around the time when some companies settled independently
with Mateel.  When the multi-company consortiums settled later on, they
used the same 300 ppm value.  Having done some similar assessments, I
agree with your point that 300 ppm is a conservative number.

It's interesting to note that what drove this case is that the Prop 65
MADL number for lead is so low-- half of one millionth of a gram per day
(probably less than what's in a typical calcium supplement).  So when a
commonly used chemical has a low prop 65 MADL, there could be Prop 65
exposure issues, although there are published Prop 65 MADL values for only
11 substances today.  Prop 65 No Significant Risk Levels (NSRLs), of which
there are 100's, are calculated using a different exposure model-- so
substances with low NSRL values are not as likely to be an issue.

Regards,
Joe Johnson
Environmental Regulatory Manager
Microsoft

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