Some plants produce substances or allergenic materials which can harm humans or animals who come in contact with them.

SelecTree provides toxicity information under the attribute "Health Hazard" and is defined as follows:

• Allergy = the tree may cause an allergic reaction due to its airborne pollen or a chemical it extrudes onto its bark or leaves.
• Irritant = some aspects of the tree such as plant hairs, oils or odor may cause irritation to the skin, eyes, nose or throat.
• Poisonous = a substance produced by the tree can cause injury, illness or death.

Useful links:

SelecTree Attribute Search - see "Health Hazard"
Poisonous Plant Information Database - Cornell University
Poisonous Plants - Univ. of Pennsylvania
Poisonous Plants by Degree of Toxicity - Perdue Univ.
Allergy Free Gardening
Equine Health: Poisonous Plants
All About Horses: Toxicology