Monitoring means measuring to show that control is adequate. It has nothing to do with the state of a worker's health.
Monitoring is appropriate:
Monitoring can also indicate the spread of contamination, eg surface wipes.
Screening, eg colorimetric detector tubes, meters, provides indicators of worker exposure only.
Personal air monitoring measures how much of a substance the worker inhales.
Biological monitoring measures how much of a substance has entered the body.
Further information has details of organisations that can help with monitoring.