It harms your health. Petroleum contains hydrocarbon compounds and are toxic to human body. If the vapour of these compounds was inhaled , it will cause dizziness instantaneously. In long term, it will hurt your lung, your brain and may cause cancer as some compounds (e.g. benzene) in petroleum can induce cancer.
Therefore, vapour recovery system was installed in petro stations in HK to recover the petro vapour emitted in the petro replenishment process.