I don't see any conflict of interest or illegality in doing these kinds of monitoring in-house. It is acceptable if done correctly.  

The reasons for outsourcing them are: you get a competent and experienced person to do it; you get availability of the right equipment which is kept calibrated; it is less time and trouble and risk than to train in-house people and take them off other work.

A convenient way of doing personal monitoring is that you have a stock of equipment (personal noise monitors, air pumps) that you organise the wearing of and record-keeping for (who wore it, when, doing what work in what area) and then send them off to a lab for analysis.