Hi Boyd:

One thing that should be taken into account if the tires are being used for vegetable gardens is the possible potential of the leaching of heavy metals and polynuclear aromatic hydrocarbons (PAHs) from the tires.

The tires should be lined with plastic to prevent the possibility of contaminating the the soil and thus the vegetables.

This actually applies to any framework used in constructing or erecting raised beds where food for human consumption is to be grown.

Cornell