Whilst the materials on this page that are not known to have any significant health risks associated with them, they cannot all be classed as completely hazard free.
The largest single issue is the slipperiness of the mix- it can be very easy to slip over on. It is therefore essential to consider this when planning any activities involving slime, including clean up and washing off as the most significant effects will be found on a smooth surface such as a bath!
The materials are relatively fine powders, and as such create nuisance dust. It is therefore worth wearing a dust mask if handling large quantities. Likewise if this dust gets into eyes it can cause irritation.
Guar gum is not hazardous when eaten, however it can be used as a laxative, so it would probably be unwise to consume large quantities.
Borax (Sodium Tetraborate) is not safe for consumption, and should therefore be handled and used carefully so as to avoid ingestion, although in the quantities actually present in slime it should not cause any concern.
In addition to the specific information above, general safe chemical handling practice will further reduce the risk of any issues, in particular;
Store materials in sealed, well marked, containers in an area out of reach of children and pets
Wash thoroughly before and after handling these materials
Dispose of excess/ used materials/ empty packaging responsibly
Clean up any spills quickly- Avoid using water to stop the area becoming slippery
Do not use utensils/ containers/ cleaning cloths that will be later used for food
Supervise children whilst making or using the slime
In these instructions the quantities I give will make 1 litre of stringy slime. Quantities can therefore be easily adjusted to suit smaller or larger batches to suit your needs.