SOAEL and LOAEL: what the noise policy statement means

SOAEL and LOAEL are effect levels, not numbers, and national policy declines to attach numbers to them on purpose. The Noise Policy Statement for England defines the lowest observed adverse effect level as the level above which adverse effects on health and quality of life can be detected, and the significant observed adverse effect level, SOAEL, as the level above which those effects become serious enough to matter in policy terms. Everything an assessor does with those two acronyms is an argument about where, on this site, for this source, at this time of day, those levels fall.

The effect levels and what each one means

The Noise Policy Statement for England, published by Defra in March 2010, sets three aims for noise management: avoid serious adverse impacts on health and quality of life, mitigate and minimise adverse impacts, and where possible contribute to improvements.

Those aims map onto a hierarchy of effect levels.

The Planning Practice Guidance on noise carries the same structure with a description of perception and the action expected at each stage. The one to hold onto is the middle row. Between LOAEL and SOAEL, the policy answer is not refusal. It is mitigation to reduce the effect to a minimum, taking account of what is reasonable.

Why national policy avoids fixed numbers

A single national figure would have to cover a motorway, a nightclub, a rooftop chiller and an anaerobic digester, at receptors ranging from a rural cottage to a flat over a high street.

Context changes the answer in ways a number cannot capture: the character of the sound, its tonality and impulsivity, whether it is continuous or intermittent, the existing background level, the time of day, and the expectations that go with the area. A rating level that would pass without comment in a city centre will generate complaints in a village.

So the Statement sets the framework and leaves the values to the assessor, which places the burden of justification exactly where the expertise sits. That is a defensible policy choice. It is also why noise evidence is argued over more than most technical disciplines.

How assessors derive site specific values

In practice the values come from standards applied to the circumstances, with reasoning shown.

• BS 8233:2014 internal guideline values, commonly 35 dB LAeq,16hr for living rooms in the day and 30 dB LAeq,8hr for bedrooms at night, with external amenity guidance for gardens.
• WHO night noise guidance, used to support night-time positions.
• BS 4142:2014+A1:2019 for sound of an industrial or commercial character, where the rating level is compared against background and context is assessed.
• ProPG, the joint professional guidance on planning and noise, which sets a risk-based approach to new residential development near transport noise.

The argument to make in the report is explicit: here is the source, here is the standard that applies to it, here is the value adopted as LOAEL and as SOAEL, and here is why. An assessment that names a SOAEL without saying where the figure came from invites an objector's acoustician to propose a lower one.

Where glazing and ventilation are used to achieve internal levels, say what the scheme depends on. A Noise Impact Assessment that reaches 30 dB in bedrooms only with windows shut needs to explain how the room is ventilated and how it avoids overheating, and noise insulation and glazing specifications in conditions covers how that gets secured.

Applying the hierarchy to a decision

The hierarchy tells the decision maker what kind of response is expected, and that is its value in an officer report.

1. Establish the existing acoustic environment with measurements representative of the receptor and the period.
2. Predict levels at receptors for the proposal, stating the assumptions and the plant specification.
3. Compare against the adopted LOAEL and SOAEL for the source and period.
4. Where effects fall between the two, set out mitigation and show the residual position.
5. Where effects exceed SOAEL, explain why the development should still proceed, or change the scheme.
6. Carry the mitigation into conditions that are precise enough to enforce.

Step five is where applications are lost. Exceeding SOAEL is not a bar in every case, since policy requires such impacts to be avoided while other planning considerations still apply. But an application that concedes an effect above SOAEL and offers nothing beyond a management plan is asking the authority to do the reasoning for it.

Where the terms turn up in appeals

Inspectors use the vocabulary routinely, most often to test whether the assessor justified the values adopted rather than to substitute their own. Decisions of that kind are indicative of how the issue has been approached rather than binding, and they are worth reading for the reasoning rather than the outcome.

Two patterns recur. Where an applicant sets SOAEL high and the council's environmental health officer sets it lower, the inspector generally looks for which value is better supported by the standards and the measured context. And where mitigation depends on residents keeping windows closed, the treatment is usually sceptical unless alternative ventilation is designed in.

Deliveries and night-time servicing are their own argument, since a short loud event is assessed differently from a steady level, and noise from deliveries and HGV movements at night looks at that in detail.

Common misuse of the terms

Four errors come up often enough to list. Treating LOAEL as a pass mark, when it is the point at which adverse effects start. Quoting a SOAEL taken from another site without adjusting for source and context. Using internal guideline values as though they were external limits. And applying the framework to a source that has its own assessment method, then reporting both and choosing the more favourable answer.

There is one honest uncertainty worth stating in any report. The boundary between an adverse effect and one above SOAEL is a judgement about people, informed by measurement, and two competent acousticians can land in different places. Saying so, and showing the reasoning, is more persuasive than presenting a derived number as though it were a published limit.

Where the source is a permitted installation, the noise question may be split between regimes, and statutory nuisance and planning sets out what a planning condition can and cannot fix.

Environmental health officers often set their own values in consultation responses, and an early conversation at pre-application stage costs less than an argument at committee. Ask which standards the officer expects to see applied before the survey is commissioned, because the measurement positions and periods follow from that.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

PlanScope drafts noise documents structured to the policy hierarchy, and councils expect a suitably qualified acoustician to take responsibility for the measurements and values. Enter your postcode on PlanScope to see which reports your site needs.

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