The chief poison is cicutoxin, an unsaturated aliphatic alcohol that is most concentrated in the roots. Upon human consumption, nausea, vomiting, and tremors occur within 30-60 minutes, followed by severe cramps, projectile vomiting, and convulsions. There are occasional long-term effects, like retrograde amnesia. Ingestion of water hemlock in any quantity can result in death or permanent neurological damage of the central nervous system.
Hemlock's distinguishing characteristics are that it requires a more consistent supply of water than Lomatium or Osha, and Lomatium species tend to prefer dry rocky soils devoid of organic material. Lomatium roots have a delicate rice-like odor, unlike the musty odor of Hemlock, with finely divided, hairlike leaves in most Lomatium species. Lomatium species tend to produce yellow flowers, but some species are white flowered and closely resemble Poison Hemlock. If the plant is growing on a hillside in dry, mineral soil far away from a source of water and has umbells of yellow flowers, its likely a Lomatium. It the plant is growing in an area near water in consistently moist soil, is tall (0.75-2m), has purple splotches on the main stem, and is heavily branched with small umbels of white flowers, it is probably Hemlock and should be avoided.
Osha does not do well in overly moist soils since it is a species dependent on mycorrhizal fungi to survive, but there are areas where Osha and Poison Hemlock can be found only a few feet from each other. Poison Hemlock lacks the "spicy celery" odor of Osha, and is easily distinguished from it due to the absence of hairlike dead leaf material present on the root crown of Osha roots. Poison Hemlock roots in many cases have no discernible odor, and are typically heavily branched rather than carrot-like, but this is not always the case. The plants themselves smell musty or "mousy", and in most instances will have purple blotches or shading on the lower stem of the plant if the plant is fairly mature, but again, this is not always the case.
In the Mountain West of North America, poison hemlock has become well established and invasive, and can be found in remote mountain areas anywhere water is present or soils are persistently moist. It is often found growing in the same habitat and side by side with Osha and Lomatium species, useful medicinal relatives in the Parsley family which Hemlock closely resembles, and can be very difficult to distinguish from Lomatium (an important historical food plant of Native Americans known as Biscuit Root).
A useful trick to determine whether a plant is poison hemlock rather than fennel, which it resembles, is to crush some leaves and smell the result. Fennel smells like anise or liquorice, whereas the smell of poison hemlock is often described as mouse-like or musty. Considering the high toxicity of poison hemlock, if the plant cannot be identified it must be discarded. Coniine can be absorbed through the skin, and it is well advised to wash your hands immediately after handling this plant and avoid touching your eyes or mouth if you have recently handled or come into contact with Poison Hemlock, or if you have crushed the leaves of this plant in your hand to perform a "smell test".
Poison hemlock is sometimes confused with water hemlocks in the related genus Cicuta, but are readily distinguished by the less finely divided leaves of the latter; the leaf veins of poison hemlock also run through the tips of the teeth, but those of the water hemlock run through the notches in between the teeth. The poison hemlock's root is long, white, and fleshy and is usually stringy and heavily branched, but can be carrot-like and unbranched in younger specimens of Conium. Water hemlock's roots are made up of several tubers, and are typically chambered, and excude a yellow, rank, highly toxic sap that contains cicutoxin.