19 June 2026 · 4 min read

What 'compliant' actually means: the daily-average and 2× half-hourly rule

Enforcing the Clean Air Regulations 2014 — Part 1 of 5. A series for regulators and compliance staff, drawn from the DOE CEMS Guidelines (Version 8, 2025), which we helped develop.

“Is this facility compliant?” sounds like a yes/no question, but the answer rests on a precise test that’s easy to half-remember and get wrong. Under the Environmental Quality (Clean Air) Regulations 2014 (CAR 2014), continuous-monitoring compliance is a two-part rule — and both parts must hold.

The rule: Regulation 17(3)

For continuous emission monitoring, the Emission Limit Values (ELVs) are complied with if, over the operating period within any one calendar year:

  1. no daily average exceeds the ELV, and
  2. no half-hourly average exceeds two (2) times the ELV.

Two thresholds, two averaging periods, one verdict. Pass both and the facility is compliant for that period; breach either — even once — and it is not.

A worked example

Say a facility’s daily ELV for total particulate matter is 150 mg/m³. Then:

  • The daily limit is 150 mg/m³ — every daily average must stay at or below it.
  • The half-hourly limit is 2 × 150 = 300 mg/m³ — no half-hour average may exceed it.

So:

  • A half-hour averaging 280 mg/m³ is within the half-hourly limit (≤ 300). On its own, not an exceedance.
  • A day averaging 160 mg/m³ is an exceedance of the daily ELV — non-compliant, even though no single half-hour may have looked dramatic.
  • A half-hour spiking to 320 mg/m³ breaches the 2× half-hourly limit — an exceedance, regardless of where the daily average lands.
Regulation 17(3): the two-part compliance test A chart of half-hourly averages across one day. The daily ELV of 150 and the half-hourly ceiling of twice the ELV (300) are shown as limit lines. A normal half-hour at 280 stays below the ceiling, while a spike to 320 breaches it. 300 150 0 Half-hourly averages across one operating day mg/m³ Half-hourly ceiling = 300 (2× ELV) Daily ELV = 150 Above 2× ELV — exceedance At/below ELV — daily-average compliant zone 280 ✓ 320 ✗ breach Regulation 17(3): two thresholds, one verdict
Half-hourly averages may sit between the ELV and the 2× ceiling — but a day averaging 160 mg/m³ still fails the 150 daily ELV. Both parts must hold.

Why two thresholds?

The structure is deliberate. The daily average governs sustained performance — it stops a facility running persistently high. The 2× half-hourly ceiling caps short-term behaviour — it tolerates brief, normal process variation but draws a hard line on spikes. Together they prevent both “always a bit over” and “occasionally way over.”

How the daily average is built — Reg 17(4)

The daily figure isn’t measured directly; it’s derived. Under Reg 17(4), for each calendar day the owner or occupier calculates the daily mean value (relating to the daily operating time) from the half-hourly mean values. In other words, half-hourly averages are the building blocks, and the daily average is computed from them.

That’s why data quality at the half-hourly level matters so much for enforcement — if the underlying half-hours aren’t valid, the daily figure built on them can’t be relied on. (We cover exactly when a reading counts as valid in Part 2 of this series.)

What counts as an “excess emission”

An excess emission is an exceedance of the applicable ELV as shown by a valid measurement, reported in the correct units and averaging period. The emphasis on valid is the point: enforcement stands on data that has passed the validity rules — not on raw, unqualified numbers. An exceedance also triggers specific notification duties (the 24-hour rule), which we’ll cover in Part 3.

The yardstick, for regulators

Assessing a year’s data comes down to two passes:

  • Check every daily average against the ELV; and
  • Check every half-hourly average against 2× the ELV.

A single valid exceedance of either threshold is non-compliance for that period — and the basis for the reporting and enforcement steps that follow.

Operate a regulated facility and want to understand how your data will be assessed? Talk to us — we help operators read their numbers the way the regulator does, using the DOE CEMS Guidelines we helped write.

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This article is general guidance, not legal advice. For the precise legal position, refer to the Environmental Quality (Clean Air) Regulations 2014, the EQA 1974, and the DOE CEMS Guidelines.

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