Why monitor coverage matters
Each annual record says how much of the year its monitor worked. A count from a monitor that worked for half the year can miss half of the events.
What to do
Check monitor operation before you compare counts. Treat counts below 90% with care, and never read a missing figure as zero.
What the figure means
Monitor operation, also called operability, is the share of the reporting period when the event duration monitor was recording. Companies report it in the annual return beside each spill count.
Why we use 90%
We use 90% as a simple cut-off for like-for-like comparisons and for lists of outlets with zero spills. Above it, most of the year was watched. It is our reading rule, not a legal standard.
How often coverage is low
In the 2025 return, 1,039 of the 14,303 outlets had a monitor that worked for less than 90% of the year, and 35 gave no figure. Their counts are still shown, with a note.
How our pages handle it
Outlet pages mark low-coverage years in the history table and in amber in the chart. Reports and trend sections leave those outlets out of like-for-like changes between years.