Garden chemicals or pesticides, such as insecticides, fungicides and weedkillers, are perfectly safe, providing they are used in exactly the way described on the container or packaging.
The RHS Advisory Service has written several advice profiles on garden chemicals; including,
Pesticide legislation
In recent years, due to pesticide legislation, several garden chemicals have been removed from sale and can no longer be used in the garden, house or greenhouse.
If you have any of these withdrawn chemicals, or excess undiluted chemicals you no longer want to use, they should be disposed off in the correct manner. Contact your local waste disposal authority if you are unsure of what to do.