I think your definition of "toxic" may mean it doesn't contaminate soil or water, but as far as humans goes;
How can propane affect my health?
Propane is an asphyxiant, which cuts off oxygen to the body. Exposure to very high concentrations of propane can cause death by suffocation from lack of oxygen.
Exposure to high levels of propane can cause cardiac arrest, incapacitation, unconsciousness, or seizures. Direct skin contact with liquid propane can cause frostbite.
Exposure to lower levels of propane can cause damage to the central nervous system, lung congestion, fluid in the lungs, fatigue, decreased night vision, tunnel vision, hallucinations, dizziness, lightheadedness, nausea, vomiting, headache, euphoria, numbness and tingling of arms and legs, hyperventilation, rapid heartbeat, poor judgment, confusion, memory loss, anorexia, nosebleeds, conjunctivitis, weight loss, and skin irritation and discoloration.
I'd have to say TOXIC.