What sort of wall is this?

Last night, as I was doing some electrical work, I decided I needed to open a wall up. So I pulled out my utility knife and started cutting out a section; to my surprise, it wasn't drywall (or plaster), but rather some sort of cardboard-like substance. It's about as thick as normal drywall, held up by drywall nails. What is this, and how long ago were people using it? Are there any potential health hazards I should be aware of when working on/around it?
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David Gale wrote:

Probably homasote, or some other pressed celulose board. If it's making a lot of dust, wear a mask.
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David Gale wrote:

Homosote...It makes a fairly good bulletin board material too....See:
http://www.hstech.org/howto/material/homosote.htm
HTH,
Jeff
--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
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