Site conditions, and thus atmospheric chemical conditions, may change following the initial characterization. For this reason, monitoring should be repeated periodically, especially when:
- Work begins on a different portion of the site.
- Different contaminants are being handled.
- A markedly different type of operation is initiated (i.e. barrel opening as opposed to exploratory well drilling).
- Workers are handling leaking drums or working in areas with obvious liquid contamination (i.e., a spill or lagoon).