Dust assessment for demolition and earthworks

Construction dust is assessed by risk category rather than by predicted concentration, and that is the single thing most developers get wrong when they read their first assessment. Nobody models a dust plume from a demolition site. The recognised method sorts the works into four activities, rates the dust each can generate, rates how sensitive the surroundings are, combines the two into a risk category, and then prescribes mitigation matched to that category.

A construction dust assessment therefore concludes, almost every time, that the residual effect is acceptable once mitigation is applied. The value of the work is in the mitigation schedule, not in the conclusion.

How the risk based method works

Four activities are assessed separately, because they behave differently and often happen at different times:

• Demolition, which produces short, intense releases, especially from structures above ground level.
• Earthworks, meaning excavation, cut and fill, soil stripping and stockpiling.
• Construction, covering the building works themselves, cutting, grinding and material handling.
• Trackout, which is dust carried onto the public highway on vehicle wheels and then resuspended.

For each, the assessment assigns a dust emission magnitude, assesses the sensitivity of the surrounding area, and combines them into a risk category running from negligible through low and medium to high. The mitigation that follows is set by the category.

The assessment also covers two effect types: dust soiling of surfaces, which is the nuisance complaint, and raised particulate concentrations, which is the health question. Sensitivity is assessed separately for each, because a car park is sensitive to soiling and not to health effects, while a hospital is sensitive to both.

Dust emission magnitude by activity

Magnitude is assigned from the scale and nature of the works. The thresholds in the recognised guidance run broadly as follows, and they are worth checking against the current version before use.

Volume, not floor area, drives the demolition and construction ratings. A single-storey warehouse of substantial footprint can rate lower than a smaller but taller building, which sometimes surprises people reading the assessment for the first time.

Two site-specific factors adjust the rating: the dust potential of the materials involved, and whether the works fall in wetter or drier months. Concrete crushing in July is not the same proposition as timber frame erection in February.

Sensitivity of the surrounding area

Sensitivity combines three things: how sensitive the receptors are, how many of them there are, and how far away they sit.

Distance bands typically step at 20, 50, 100, 200 and 350 metres from the site boundary or, for trackout, from the route used by construction traffic. Receptor counts matter because twenty dwellings within 20 metres produce a different rating from one dwelling at the same distance.

Highly sensitive receptors include homes, schools, hospitals and care settings for health effects, and homes, painted surfaces, horticulture and certain commercial premises for soiling. Ecological receptors are assessed separately where a designated site lies within the distance bands, because dust deposition on vegetation is a different mechanism.

Existing background particulate concentration feeds into the health limb, taken from published national mapping. A site in an area already close to the objective is rated more sensitive than the same site in clean air.

For trackout, assess along the actual haul route rather than around the site, and extend the assessment to the point where the route reaches a properly surfaced road. This is the part most often left out.

Assigning a risk category

Magnitude and sensitivity combine in a matrix to produce the risk category for each activity and each effect type. The output is therefore a small grid, not a single answer: demolition may be high risk for soiling and medium for health, while construction is low for both.

Present it as a table. An assessment giving one overall risk category for the whole project has lost the information that determines the mitigation.

The category is assigned before mitigation. That is deliberate, because the whole purpose of the exercise is to set the level of control. A high risk site with the mitigation applied is expected to have no significant residual effect, and the assessment should say so plainly rather than implying the risk category itself is the conclusion.

Mitigation that follows the category

Measures scale with the category. The commonly required set, from the baseline upwards:

1. Site management: a named responsible person, a complaints log with response times, and records kept and available for inspection.
2. Site boundary: solid screens or barriers appropriate to the works, and clean site hoardings.
3. Water suppression at cutting, crushing, screening and demolition, with a supply and a plan for winter operation.
4. Covered or damped stockpiles, and stockpiles sited away from the boundary and downwind receptors where possible.
5. Wheel washing and a hard surfaced haul route, with road sweeping at the site exit.
6. Sheeting of vehicles carrying dusty loads.
7. No bonfires, no uncontrolled burning of site waste.
8. Restrictions on drop heights from excavators and conveyors.
9. Enclosure of crushing and screening plant on higher risk sites.

Most of these end up in the Construction Management Plan secured by condition, and the haul route measures sit alongside the arrangements in a Construction Traffic Management Plan. Writing them into both documents consistently avoids a discharge query.

Monitoring, trigger levels and action

Monitoring is expected on medium and high risk sites and is sometimes conditioned regardless.

Three techniques, used together or separately. Daily visual inspection of the site boundary and the haul route, logged. Dust deposition gauges or directional sticky pads, which measure soiling over a period and are useful for attributing complaints. And real-time particulate monitors at the boundary, which give a live reading and can trigger an immediate response.

Real-time monitoring is where trigger levels come in. A boundary monitor with an agreed threshold and an agreed action protocol turns dust control from a paper commitment into an operational one. The protocol should say what happens at the trigger: check the wind direction, identify the activity, apply additional suppression, and stop the activity if the level does not fall.

Set the baseline before works start. A monitor installed on day one of demolition records everything and attributes nothing, because there is no pre-works reference.

What the management plan must contain

A dust management plan that satisfies an environmental health officer covers:

• The risk categories assigned, by activity and effect type.
• The mitigation measures, mapped to the category that requires them.
• The responsible person and the chain of command for stopping work.
• Monitoring locations, equipment, trigger levels and reporting frequency.
• The complaints procedure, with response times and a record format.
• Arrangements for dry weather and high wind, including a threshold wind speed at which dusty operations stop.
• Inspection and maintenance of suppression equipment.
• Arrangements for informing neighbours before particularly dusty phases.

That last item does more to reduce complaints than most of the engineering measures. A letter a week before demolition begins, with a contact number that someone answers, changes how neighbours respond to the dust they see.

Where the site also generates construction noise, the two assessments should share the same programme and the same receptor list, and the limits of screening as a control are covered in what an acoustic barrier actually achieves. Note also that a permission and its conditions do not displace the council's nuisance powers, as explained in statutory nuisance after planning permission is granted.

An Air Quality Assessment usually carries the construction dust chapter alongside the operational assessment, and the two should agree on receptor locations.

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