Hi,
You have an excellent question; it's always good to exercise some precaution when working with any unfamiliar materials. When a scientist has a question about the safety of materials to be used in an experiment, the first thing to do is to check the material safety data sheet, or MSDS, from the product supplier.
Here are MSDS’s for agar and glycerol, for example:
http://www.sciencestuff.com/msds/C1154.html
http://www.sciencestuff.com/msds/C1794.html
You can see that both products are not hazardous in normal handling. Agar is rated 0 health hazard, 1 for flammability, and 0 for reactivity. Glycerol is rated 1 for health, 1 for flammability, and 0 for reactivity. This information should reassure you that there is no significant danger is working with these chemicals.
Food grade and lab grade agar will have the same chemistry, but the lab grade product would have a higher purity. You don’t need to buy the food grade again.
You still need to follow general safety precautions when working with these materials, however. I prefer melting agar in the microwave oven. Heat it slowly, mixing the container frequently. Use a contain that is 2-3 times larger than the volume of the agar solution to minimize the chance of boilover, as agar is like a bowl of oatmeal when it reaches its boiling point when it is microwaved. The agar won’t dissolve completely until it boils, so you will have to heat it until it boils.
I hope this helps. What are you doing for your project?
Donna Hardy