Answered by Ustadh Tabraze Azam
Question: Assalam ‘aleykum.
Are tears due to fever or flu impure?
Answer: Wa alaikum assalam wa rahmatullahi wa barakatuh,
I pray that this message finds you well, insha’Allah.
No, normal tears are not filthy or impure, and they do not invalidate your ablution (wudu). [Samarqandi, Tuhfat al-Fuqaha]
What is invalidating is the exit and flow of filth, and with respect to the eyes, fluid which exits due to an illness or injury. The sign of this is that the exiting is usually accompanied by pain.
As for allergies and the like, they do not affect your wudu unless fluid exits in the manner described.
Please also see: Could You Please List All the Nullifiers of Ablution According to the Hanafi school? and: Do Tears from Onion Fumes Break Wudu?
And Allah alone knows best.
Wassalam,
Tabraze Azam
Checked & Approved by Shaykh Faraz Rabbani