Slatmill safety
A slatmill is exercise equipment, and the same rules apply as to any other: supervision, progression, and knowing when to stop.
Never unattended
A dog on a mill should never be left alone, and should never be attached to it. If a dog stumbles or wants to stop, it must be able to.
Heat
Dogs shed heat through panting and their paws, not by sweating. Indoor sessions in warm rooms build heat quickly. Provide water, keep the space ventilated, and stop at the first sign of heavy laboured panting.
Joints and age
Growing dogs should not be doing sustained conditioning work; nor should dogs with undiagnosed lameness. If a dog is being conditioned after injury, that belongs with a veterinary physiotherapist rather than a mill in a garage.
Surface and maintenance
Check bearings, slat fixings and belt tension regularly. A slat that works loose at speed is the main mechanical risk these machines have.