Symptoms generally start within a couple of hours of ingestion.
The entire palnt is poisonous, but the toxicity within parts of the plant tend to change with the time of year. The leaves being more poisonous in the spring, and the seeds more so in the fall. Human poisonings are generally due to mistaking hte plant for edible or medicinal plants such as wild carrot, parsnip. or anise. It is quite common out west, I run across it a lot when I am collecting herbs. The other hemlock is a tree in which hemlock essential oil can be produced.