That depends on what you mean by "affect external interrupt line"

Humidity does not affect the internal operation of the 89C51; however, changes in humidity could result in effects on the signal that your system applies to the 89C51's pin. If these changes take the signal outside the required operating conditions, then that will affect the behaviour of the 89C51!


Air humidity could also affect the operation of the circuitry that you have inserted between the sensor and the 89C51's pin - especially if that circuitry is poorly designed and/or poorly laid-out...

EDIT:

Similarly, temperature could affect the circuitry and/or the sensor - and many other things!