A typical ear can handle up to around 140dB for a short period of time, and around 100dB for a long period.
Super loud, high performance fans can be around 50dB, normal fans can be around 30-35dB, and the quiet ones which I said can be between 18 and 20dB.
Noise is better measured in Sones, as some frequencies seem louder than others, and measuring for Sones takes this into account.