I stand corrected. Now I see my mistake. Given that line:
humidity=37;40:60;30:70
I thought this is supposed to mean
WARN=40, CRIT=60, MIN=30, MAX=70.
But it actually means
WARN_lower=40
WARN_upper=60
CRIT_lower=30
CRIT_upper=70
MIN=unset
MAX=unset
Sorry for the confusion.