Pollution hotspots methodology

Friends of the Earth has analysed data and identified that 27% of neighbourhoods in England classify as "pollution hotspots" for nature and wildlife.

Top 10 nature pollution hotspots by constituency

In this analysis we refer to "neighbourhoods". These are Lower Layer Super Output Areas (LSOAs), which are geographic areas with similar population sizes that were primarily designed for the Census but have widespread use for a range of spatial data. LSOAs have an average population of 1,500 people or 650 households. 

Identifying nature pollution hotspots

Air, water, light and noise pollution data was assessed within each neighbourhood in England to determine whether pollution levels breached thresholds deemed to be damaging to nature and wildlife.  

Using the 4 pollution categories, each neighbourhood had pollution levels above thresholds for between 0 and 4 types of pollution. We categorised nature and wildlife pollution hotspots as those where pollution levels for all 4 types of pollution were above thresholds deemed damaging to wildlife and nature, ie where air, water, noise and light pollution are all likely to be negatively impacting on the biodiversity of neighbourhoods.

The data and thresholds assigned to each type of pollution and the sources of data used for each type of pollution are provided below.

Pollution types included

Air pollution

Neighbourhoods’ air pollution thresholds were crossed when nitrogen dioxide (NO₂) and particulate matter 2.5 (PM_2.5) were both above World Health Organization (WHO) guidelines set to protect human health: 10 ug/m³ for NO₂ and 5 mg/m³ for PM_2.5. This used Defra background air pollution concentrations at 1 km resolution that was aggregated to neighbourhoods on an area basis. There's no definitive, current information on what levels of air pollution are directly damaging to wildlife and nature. For this analysis we've assumed that air pollution damaging to humans is also damaging to nature. For more information, please see Friends of the Earth’s 2022 analysis of Defra’s modelled background air pollution data.

Water and river pollution

Water pollution hotspots were identified where waterbodies were rated ecologically "poor" or "bad," and also had more than 336 hours (equivalent to 2 weeks) of sewage spills according to 2023 data. The ecological rating of water bodies is from the Environment Agency’s own data. The 336 hours of sewage outflows was a threshold set by Friends of the Earth. Sewage data was mapped directly to waterbodies rather than neighbourhoods to ascertain the impact. Waterbodies were mapped to neighbourhoods on an area basis. Where a neighbourhood boundary covered more than one waterbody, the waterbody that occupied the largest area of the neighbourhood was associated with that neighbourhood, so that there was a one-to-one matching between neighbourhoods and waterbodies.

Light pollution

Light pollution hotspots were neighbourhoods where at least 10% of the land area of the neighbourhood was exposed to light pollution above 2 nW/cm2/sr*, above which negative impacts on ecosystems and wildlife are expected. Ecologists have found evidence of light pollution impacting negatively on migration patterns, foraging strategies, reproduction habits and communication systems. This was further linked to increased mortality and altered community structures. There's also evidence to suggest that light pollution has contributed to a 40% decline in pollinating moths in recent decades, and that light pollution is reducing nocturnal pollinator visits to flowers by 62% in some affected areas.

Noise pollution

Pollution hotspots have been identified where at least 10% of the land area of a neighbourhood has L_den noise levels** from road, rail of greater than 55 dB and 45 dB for aircraft, which evidence suggests are harmful to both nature and humans. 55 dBs is the minimum level identified in the noise data. There's no clear consensus on a threshold above which noise negatively impacts on nature, but studies have shown that noise as low as 23 dB can impact on wildlife and the higher the level of noise the more significant these impacts. Impacts include biodiversity loss (reductions in both the size of wildlife populations and the occurrence of species) and altered vocal behaviour, particularly in bird song (ie to try and be heard above the competing sounds of urban activity). For humans, exposure to noise levels above 50 dB are known to impair daily activities, sleep and communication, and WHO guidelines recommend L_den noise thresholds of 54 dB for road traffic, 53 dB for railway and 45 dB for aircraft noise.

Data sources

Air pollution: Neighbourhoods with NO₂ and PM_2.5 levels above WHO guidelines: Friends of the Earth's 2022 analysis of Defra’s modelled background air pollution data.

Water and river pollution: Ecological status of waterbodies: Catchment Data Explorer, Classifications data for England, Environment Agency and Defra. https://environment.data.gov.uk/catchment-planning/ and Sewage spills: Event Duration Monitoring - Storm Overflows 2023 (England and Wales), The Rivers Trust. https://data.catchmentbasedapproach.org/datasets/theriverstrust::event-duration-monitoring-storm-overflows-2023-england-and-wales/about. (Note: The Rivers Trust compiled this using Environment Agency data for overflows in England and data accessed from Dŵr Cymru/ Welsh Water and Hafren Dyfrdwy websites for overflows in Wales.)

Light pollution: CPRE light pollution and dark skies map (data provided on request). https://www.cpre.org.uk/what-we-care-about/nature-and-landscapes/dark-skies/englands-light-pollution-dark-skies-map/

Noise pollution: Road and rail noise: Strategic noise mapping, Defra 2019. https://www.gov.uk/government/publications/strategic-noise-mapping-2019; Aircraft noise: Aircraft Noise Map (data provided on request) https://noise-map.com/home/. 

*A measure of light radiance: nanoWatt per cm² per steradian (square radian – a method of measuring the angle of circumference on the surface of the Earth).

**An equivalent noise level calculated over a 24-hour period, with penalties applied to any evening (7-11pm) or night-time noise (11pm-7am).