The chemical n-propyl disulfide in onions makes them toxic for dogs.
In large enough amounts however it can kill a dog even though the substance isnot toxic to humans (because our bodies can absorb it).
N-propyl disulfide cannot be absorbed or processed by dogs.
Therefore it attaches itself to the outside of red blood cells within a dogs body causing them to explode.
Red blood cells are the ones that carry oxygen around the body so they can not be messed with.
The more onions a dog eats however the more n-propyl disulfide is produced in their body and the more red blood cells are destroyed.
If your dog isnot treated by a vet it will become anemic and eventually starve to death.