130To make the maintenance of the authorisation of a substance subject to proof of the lack of any risk, even a purely hypothetical one, would be both unrealistic — in so far as such proof is generally impossible to give in scientific terms since ‘zero risk’ does not exist in practice (see, to that effect, Pfizer Animal Health v Council, paragraph 145, and Alpharma v Council, paragraph 158) — and contrary to the principle of proportionality.