From a quick trawl of the web
How poisonous is the yew?
Poison is found in all parts except for the fleshy fruit.
The poison is called Taxine. The alkaloid ephedrine, as well as a volatile oil and traces of a cyanogenic glycoside, taxiphyllin, are also present. HMSO 1984
The leaves are more toxic than the seed.
(Paul Greenwood 2005)
Effect on animals
There is contradictory evidence about the effect on animals of eating yew foliage. There are many recorded instances of animals known to have died from grazing on yew leaves. There are also reports of animals eating leaves without suffering any ill effects. It is not unknown for small quantities of leaves to be added to supplement winter fodder for cattle.
Poisonous plants in Britain and their effects on animals and man published by the HMSO in 1984 is however unambiguous in its advice:
..yew should never be fed to animals
.
(Copyright © Tim Hills 2005)
Effect on humans
Fifty to one hundred grams of chopped leaves is considered fatal to adults.
A world-wide investigation in 1998 (Krenzelok et al.) shows 11,197 records of yew poisoning (from all Taxus species) in humans (96.4% in children less than 12 years old) and found no deaths. A 1992 article in Forensic Science International (Van Ingen et al.) stated that only 10 authenticated cases of fatal human poisoning by T. baccata had been recorded in the previous 31 years, and that they were all deliberate.
Krenzelok, E.P., Jacobsen, T.D. & Aronis, J. (1998) "Is the yew really poisonous to you?", Journal of Toxicology Clinical Toxicology, 36, 219-223.
Van Ingen, G., Visser, R., Peltenburg, H., Van Der Ark, A.M. & Voortman, M. (1992) "Sudden unexpected death due to Taxus poisoning. A report of five cases, with review of the literature". Forensic Science International, 56, 81-87.
(Copyright © Fred Hageneder 2005)
I observed a craftsman turning wooden spindles at a lathe. When working with yew he put on a mask with a respirator and made sure the extractor fan was working properly. He explained that yew dust should not be inhaled and that anyone working with yew wood should be aware of health and safety implications.
(Paul Greenwood 2005)
Cheers
Nick