Lowest observed adverse effect level

The lowest observed adverse effect level, usually written as LOAEL, is the level of noise exposure above which adverse effects on health and quality of life can be detected. It is a policy concept rather than a measured criterion, and it comes from the Noise Policy Statement for England, which sets out three effect levels: a no observed effect level, the lowest observed adverse effect level, and, above that, the level at which effects become material, known as the SOAEL. The three are abbreviated NOEL, LOAEL and SOAEL. National planning guidance then attaches different consequences to each.

The single most important thing to understand is that national policy sets no number for any of them. The levels are deliberately left to be established case by case, which is why two acousticians can agree on the measured figures and disagree completely about the conclusion.

Where the effect levels come from

The Noise Policy Statement for England sets three aims for noise policy: avoid significant adverse impacts on health and quality of life, mitigate and minimise adverse impacts, and where possible contribute to improvements in health and quality of life. The effect levels exist to make those aims operable.

Planning guidance carries the concept into decision making with a table of perception, likely effect and the action required, running from no observed effect through no observed adverse effect and observed adverse effect to an unacceptable adverse effect at the top. The thresholds between those bands are the effect levels.

So the structure is policy, then professional judgement, then a number. Reports that start with a number and work backwards tend to come unstuck under scrutiny.

What each level describes

The distinctions are about the nature of the effect rather than the decibel figure.

Below the no observed effect level, noise is present but produces no detectable effect at all. Between that and the lowest observed adverse effect level, noise can be heard and may change behaviour slightly, but there is no adverse effect on health or quality of life.

At and above the lowest observed adverse effect level, adverse effects begin: noticeable changes in behaviour, having to close windows or raise voices, some disturbance to sleep. At and above the SOAEL, the effects are material: regular sleep disturbance, a marked change in the use of a dwelling or garden, and consequences for health.

How they relate to measured figures

Because policy sets no numbers, practitioners adopt values from technical standards and justify them. The values most often adopted as proxies for the onset of adverse effect are the internal guideline levels in BS 8233:2014, around 35 dB LAeq,16hr in living rooms during the day and 30 dB LAeq,8hr in bedrooms at night, with individual night time maximum events assessed separately. External amenity space is commonly assessed against a guideline range in the upper 40s to mid 50s dB LAeq,16hr, applied with judgement about the character of the area.

Night time exposure is usually where the argument happens, and the guideline values for sleep in the night noise guidance of the World Health Organisation are often cited alongside the British Standard.

Two cautions. These are guideline values for internal ambient noise, not statutory limits, and adopting one as a SOAEL without reasoning is exactly the shortcut inspectors criticise. And a level that marks the onset of adverse effect in a quiet rural location is not the same level that does so beside an urban main road, which is why the assessment has to describe the existing acoustic environment before proposing any threshold.

Use in the planning balance

The practical consequence of the framework is a two stage question. Does the proposal give rise to effects at or above the SOAEL, and if so can that be avoided? If not, are effects between the two levels mitigated and reduced to a minimum?

That second limb is where most schemes are decided. A development can be acceptable with internal levels above the ideal guideline where the acoustician shows the mitigation hierarchy has been worked through: site layout, orientation of habitable rooms, building form and screening first, then the building envelope, then ventilation strategy, with mechanical ventilation as the last resort rather than the first answer.

A Noise Impact Assessment that states the adopted effect levels, explains why they were chosen for this receiving environment, and then demonstrates the hierarchy, gives an environmental health officer something to agree with. One that reports measured figures against a standard with no effect level reasoning invites a holding objection, and the approach in our note on standing advice from planning consultees applies to the response.

Limits of applying the levels to plant noise

The framework was written with transportation and ambient noise in mind, and it transfers awkwardly to fixed plant.

Commercial plant and machinery is normally assessed under BS 4142:2014+A1:2019, which compares a rating level, adjusted for acoustic features such as tonality and intermittency, with the background sound level. That method expresses outcomes as the difference between the two, with a larger excess indicating a greater likelihood of adverse impact, and it folds context into the judgement.

Mapping those outcomes onto the observed effect levels is possible but has to be done explicitly. Two reports can use the same measurements and reach different conclusions because one treats a rating level exceedance as evidence of an effect above the SOAEL and the other treats it as an indication of an adverse impact to be mitigated. Where plant noise is conditioned, the condition should reference the method, the assessment position and the rating level rather than the effect level language.

An illustrative application

Take a site for 18 flats beside a four lane urban road, with a measured free field facade level in the low 70s dB LAeq,16hr and night levels around 10 dB lower. The assessment adopts the British Standard internal guidelines as representing the onset of adverse effect, and sets a SOAEL above them with reasoning about the urban context.

The layout places bedrooms on the courtyard elevation, winter gardens to the road, and a glazing and ventilation specification achieving internal levels within the guidelines with windows closed and a defined ventilation strategy. Amenity space sits in the courtyard, where the screened level falls within the external guideline range.

The conclusion is that effects above the SOAEL are avoided and residual adverse effects on the road facade are mitigated and reduced to a minimum. The figures and the outcome here are illustrative rather than taken from a decision.

What an officer expects a report to state

Six things, stated plainly near the front of the report rather than buried in an appendix.

1. The existing acoustic environment, described and measured, with survey dates, positions and durations.
2. The effect levels adopted, the values assigned to them and the reasoning for those values in this environment.
3. The standards and guidance relied on, by edition.
4. The predicted exposure for each receptor, internal and external.
5. The mitigation hierarchy worked through in order, with what was rejected and why.
6. The residual position against each effect level, and the conditions proposed to secure the mitigation.

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