This page details the methodology for calculating progress towards the PM2.5targets. The results of the calculations showing progress towards the targets can be found here.

Please note that all references to rounding numbers refer to commercial rounding, i.e., 0.5 is consistently rounded up to the nearest integer.

Population exposure reduction target (PERT)

Calculating the Base Year (PEIbase)

The Population Exposure Indicator (PEI) for the base year is calculated as follows:

  1. Download hourly and daily PM2.5 data for England sites for the base year (2018) and two preceding years (2017 and 2016) using the UK-AIR data selector or the openair R package. The data contains both automatic hourly measured PM2.5 (the majority) and gravimetric daily measured PM2.5 for the small number of sites which use these techniques. For the small number of sites where there are two types of instrument, one producing hourly and one producing daily measurements, the annual mean based on the automatic (hourly) dataset should be given priority and used in preference to the gravimetric (daily) value. The gravimetric value should only be included where data capture for the automatic technique is less than 85%.

  2. Filter out sites which are not categorised as urban background or suburban background at the year of measurement. A list of sites and their environment types can be downloaded from UK-AIR. New sites are added over time and others decommissioned, so not all sites in this list will be operational in every year. Sites categories can also change (e.g., Warrington and Middlesborough changed classification in 2022) so the category at the time of the measurement needs to be checked.

  3. For each urban and suburban background site, filter out those that were not open throughout each baseline year. For example, when considering particulate concentrations in 2016, the sites used in calculations should not have opened or closed in 2016 itself.

  4. For each urban and suburban background site calculate the percentage of hours in each year for which there are verified data, round to the nearest integer and filter out those that do not meet the 85% data capture threshold.

  5. Calculate the annual mean concentration for each of the remaining sites using the hourly and daily data. Do this for each of the three years.

  6. For each year calculate the average value of all the sites to produce an England concentration.

  7. Calculate the average England concentration for the three years and round to two decimal places to produce the base year PEI (PEIbase).

Calculating Yearly Increments (ΔPEI)

The yearly increments (change) in PEI is calculated for every subsequent year until the target deadline (or most recent year with validated data if assessing progress), using the following steps:

  1. Download hourly and daily PM2.5 data for England sites for the year and three preceding years using the UK-AIR data selector.

  2. Filter out sites which are not classified as urban background or suburban background for all four years.

  3. Filter out sites which were not operational throughout the whole of the four year period.

  4. For each remaining site calculate the percentage of hours in each year for which there are verified data to produce the annual data capture percentage.

  5. Filter out sites which do not meet the 85% data capture threshold for three out of the four years. The remaining sites are used to calculate both the reference indicator (RIyr) and PEI for that particular year (PEIyr). There will be a different RI and PEI pair for each year, calculated using a different set of monitoring sites for each year.

  6. For each of the four years calculate the annual mean concentration from the hourly or daily data for each site. Then average the values of all sites for the year that meet the 85% data capture threshold (rounded to the nearest percent), so there is an England concentration for each year, calculated using only qualifying sites.

  7. Calculate the 3-year average for the first three years (e.g. 2016, 2017 and 2018) to obtain the RI. Then calculate the 3-year average for the latter three years (e.g. 2017, 2018, 2019) to obtain the PEI.

  8. Calculate the difference between the RI and PEI calculated in Step 7 and round to two decimal places. This is the yearly increment in PEI (ΔPEI) in µg m-3 (e.g., ΔPEI2019 = RI2019 – PEI2019).

Once the yearly increments are calculated for each year up to 2040 (or the latest year with complete, verified data if assessing progress), the following steps are taken:

  1. Sum all the yearly increments (ΔPEI) in µg m-3 from 2019 to 2040 (or latest year with verified data) to produce Δsum.

  2. Divide the summation (Δsum) by the base year PEI (PEIbase) and multiple by 100 to produce the percentage change in population exposure from 2018. Round to the nearest percent.

  3. For the PERT to be met this percentage must be 35% or greater in 2040.

Annual mean concentrations and data capture percentages for each site are published on UK-AIR, rounded to the nearest percent. However, this rounding may introduce differences in the calculations, so the raw (hourly or daily) data should be used to calculate the PEI.

The following equation summarises the calculation, where n is the year for which the increment is being calculated and is between 2019 and 2040.

Data

Annual mean PM2.5 concentrations and annual data capture rates used for calculating the PERT are tabulated below. Each site is assigned a letter key which either indicates it is used in the PERT calculations (P) or provides the rationale as to why it could not be used (A-C). Each yearly increment is calculated using four years of data (see methodology), so for a site to be included in the calculation there needs to be robust data available for this whole period. Data from sites which are not operational for the full four years or do not meet the data capture threshold for at least 3 out of the 4 years are therefore not included in that year’s calculation.

Key Reason
Increment Years (2019+)
P Used in calculations.
A Not representative of population exposure (i.e., Urban/Suburban Background) for the relevant 4 years.1
B Data capture not at least 85% for 3 of the relevant 4 years.
C Not open/active for the relevant 4 years.
Baseline (2018)
P Used in calculations.
A Not representative of population exposure (i.e., Urban/Suburban Background) for the relevant 3 years.1
B Not open throughout the baseline year (i.e., a site opened or closed during the relevant year)
C Data capture not at least 85% for that year.
1 This may be that a site is of a different classification (e.g., urban traffic), or that it was reclassified as (sub)urban background during the relevant 3/4 years. For example, Middlesbrough (MID) and Warrington (WAR) were reclassified as urban background sites in 2022, so are not used in calculations before 2026.

Start by selecting one or more PEI years using the column to the far left of the table. Then, filter the other columns in different ways to understand which sites are used for calculating progress towards the PERT for that particular year. Note that the table can be scrolled left-and-right to view more content.

The data in the table can be copied or downloaded as a CSV/XLSX file using the buttons at the top left of the table. The data downloaded will reflect the filtering applied to the table. For example, filtering the “Key” column to show “P” will mean that only data used in calculations will be downloaded.


Alternatively, hourly measured concentrations and annual mean statistics can be obtained from the UK AIR Data Selector.

Annual mean concentration target (AMCT)

Compliance with the AMCT is assessed as follows:

  1. Download hourly PM2.5 data for England monitoring sites for 2040 (or the most recent year with complete, verified data if assessing progress) using the UK-AIR data selector.

  2. For each site calculate the percentage of hours in the year for which there is verified data, rounded to the nearest percent.

  3. Filter out sites which do not have verified data for 85% or more of the hours in the year (i.e., do not meet the data capture threshold). For the small number of sites where there are two types of instruments one producing daily and one producing hourly measurements, use the hourly data for the assessment.

  4. Calculate the average concentration over the year for each site meeting the data capture threshold and round to the nearest integer.

  5. Compare these sites against the target. All sites need to have annual mean concentrations of 10 µg m-3 or below in 2040 for the AMCT to be met.