Pain which accompanies the exiting of water from the eyes is not the actual determinant in nullification of wudu. Rather, as the scholars clarify, the nullification of wudu is determined by the water exiting because of an illness or disease, and pain is generally an indication of that. [Ibn Abidin, Radd al-Muhtar]
Based on this, the tears resulting from onion fumes does not break the wudu, even if one feels stinging or pain.
