Alert level

A site-defined level, below the action level, that gives early warning of a drift from normal conditions. It is usually derived from the site's own historical data, for example the 95th percentile of a year of results, and should be reviewed at least yearly. Reaching an alert level triggers closer attention, trending and sometimes resampling, but not necessarily a full investigation. Repeated alert-level results at one location are often treated like an action-level excursion.