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We first became aware of an issue with the hydrogen sulphide (H2S) data being recorded by the analysers in our Mobile Monitoring Facilities (MMFs) during a routine audit on 21 August 2023.We are sorry that this happened and for the concern that it caused.
We immediately began an investigation. We concluded that the issue arose from a misunderstanding about the calibration procedures when we purchased the new analysers in 2016.
We have updated the calibration procedure in line with the manufacturer’s recommendations and are confident in the H2S data we have been collecting since 1 September 2023 for all MMFs.
We continue to produce monthly Air Quality reports using data recorded by analysers subject to the updated calibration procedures, which are available to view on ourAir Quality Monitoring page.
Method to adjust the historic data
We developed a methodology that would allow us to adjust the historic data. This involved using sulphur dioxide (SO2) data that was considered to be reliable. When measuring H2S the instrument used chemically converts H2S into SO2. This SO2 data formed the basis of the recalibration of historic H2S data.
This methodology was independently reviewed by a peer review group led by Defra’s Chief Scientific Adviser. The peer review group consisted of experts from industry and academia with a range of knowledge and experience spanning instrument calibration and air quality measurement. The peer review concluded that the method we developed could be used to adjust the historic data where existing calibration slopes are available for SO2.
The hydrogen sulphide Calibration Expert Group Report was published on 8 August 2024 and can be viewed here.
We have shared the adjusted data with the UKHSA which has reviewed its health risk assessments and produced a report based on the adjusted data. You can view the report on the right hand side of the page.
Understanding the adjusted historicdata
We hold SO2 calibration slope data for all MMFs from October 2021 and from May 2021 for MMF9. We have adjusted the data sets and are now sharing the (original) historic data, adjusted data, and the associated uncertainty values (95 % confidence) for the adjusted historic data.
For the full range of values recorded, please refer to the data here. This spreadsheet shows all of the data collected from the MMFs from 2021, the calculations and the resulting adjusted data with margins of error. You can find more information about how these figures are derived from a combination of the adjustment methodology and uncertainty budget which appears in annex B of the peer review report.
The difference is instrument-specific and involves a combination of slope adjustment, baseline adjustment and adjustments taken from H2S conversion efficiency. These adjustments are calculated with their uncertainty expressed as a percentage to a 95% confidence level.
The peer review group recommended we express the associated uncertainty with the adjusted data. We are also considering a further recommendation from the peer review group that we describe uncertainty in the data for all parameters we measure during investigative atmospheric air monitoring.
We have produced 4 charts, one for each MMF, to show the adjusted data set from 2021 to August 2023, and the data recorded using the updated calibration procedure between September 2023 and July 2024. The charts show you how the adjusted data compares to the original historic data. Please click on each of the links below to view an enlarged version of the graphs.
TheHistoric H2S data (Blue Line) is displayed as monthly arithmetic means. The historic data underwent the data ratification process and was adjusted for drift and zero offset using SO2 precision check data (based on the original calibration procedure). This line is the data that had previously been published before we identified an issue with the H2S data.
The Adjusted H2S data (Red Line) is displayed as monthly arithmetic means. This data has been adjusted using the independently peer reviewed method, which involves use of the available SO2 calibration slopes.
The Uncertainty Band(Yellow Fill), is the range between the upper and lower uncertainty values for adjusted H2S data*.
For example, for MMF 1 on 30 July 2022, the original value at 10:00am was 0.8, the adjusted value of this data is 2.26 with a lower uncertainty value of 1.74 and a higher uncertainty value of 2.78.
The upper dotted line shows the H2S Uncertainty upper value (95% confidence). This is the upper uncertainty percentage represented as a concentration with a confidence level of 95%.
The lower dotted line shows the H2S Uncertainty lower value (95% confidence). This is the lower uncertainty percentage represented as a concentration with a confidence level of 95%.
The green line represents H2S data recorded after September 2023 displayed as monthly arithmetic means. This data was recorded using the updated calibration procedure. It has undergone the data ratification process and has been adjusted for drift and zero offset using H2S precision check data.
* The uncertainty values are based on an hourly average at a limit value of 100 parts per billion (ppb) and an analyser range of 2000 ppb and defined oxidiser efficiency; and are displayed as monthly arithmetic means.
Introduction
We first became aware of an issue with the hydrogen sulphide (H2S) data being recorded by the analysers in our Mobile Monitoring Facilities (MMFs) during a routine audit on 21 August 2023.We are sorry that this happened and for the concern that it caused.
We immediately began an investigation. We concluded that the issue arose from a misunderstanding about the calibration procedures when we purchased the new analysers in 2016.
We have updated the calibration procedure in line with the manufacturer’s recommendations and are confident in the H2S data we have been collecting since 1 September 2023 for all MMFs.
We continue to produce monthly Air Quality reports using data recorded by analysers subject to the updated calibration procedures, which are available to view on ourAir Quality Monitoring page.
Method to adjust the historic data
We developed a methodology that would allow us to adjust the historic data. This involved using sulphur dioxide (SO2) data that was considered to be reliable. When measuring H2S the instrument used chemically converts H2S into SO2. This SO2 data formed the basis of the recalibration of historic H2S data.
This methodology was independently reviewed by a peer review group led by Defra’s Chief Scientific Adviser. The peer review group consisted of experts from industry and academia with a range of knowledge and experience spanning instrument calibration and air quality measurement. The peer review concluded that the method we developed could be used to adjust the historic data where existing calibration slopes are available for SO2.
The hydrogen sulphide Calibration Expert Group Report was published on 8 August 2024 and can be viewed here.
We have shared the adjusted data with the UKHSA which has reviewed its health risk assessments and produced a report based on the adjusted data. You can view the report on the right hand side of the page.
Understanding the adjusted historicdata
We hold SO2 calibration slope data for all MMFs from October 2021 and from May 2021 for MMF9. We have adjusted the data sets and are now sharing the (original) historic data, adjusted data, and the associated uncertainty values (95 % confidence) for the adjusted historic data.
For the full range of values recorded, please refer to the data here. This spreadsheet shows all of the data collected from the MMFs from 2021, the calculations and the resulting adjusted data with margins of error. You can find more information about how these figures are derived from a combination of the adjustment methodology and uncertainty budget which appears in annex B of the peer review report.
The difference is instrument-specific and involves a combination of slope adjustment, baseline adjustment and adjustments taken from H2S conversion efficiency. These adjustments are calculated with their uncertainty expressed as a percentage to a 95% confidence level.
The peer review group recommended we express the associated uncertainty with the adjusted data. We are also considering a further recommendation from the peer review group that we describe uncertainty in the data for all parameters we measure during investigative atmospheric air monitoring.
We have produced 4 charts, one for each MMF, to show the adjusted data set from 2021 to August 2023, and the data recorded using the updated calibration procedure between September 2023 and July 2024. The charts show you how the adjusted data compares to the original historic data. Please click on each of the links below to view an enlarged version of the graphs.
TheHistoric H2S data (Blue Line) is displayed as monthly arithmetic means. The historic data underwent the data ratification process and was adjusted for drift and zero offset using SO2 precision check data (based on the original calibration procedure). This line is the data that had previously been published before we identified an issue with the H2S data.
The Adjusted H2S data (Red Line) is displayed as monthly arithmetic means. This data has been adjusted using the independently peer reviewed method, which involves use of the available SO2 calibration slopes.
The Uncertainty Band(Yellow Fill), is the range between the upper and lower uncertainty values for adjusted H2S data*.
For example, for MMF 1 on 30 July 2022, the original value at 10:00am was 0.8, the adjusted value of this data is 2.26 with a lower uncertainty value of 1.74 and a higher uncertainty value of 2.78.
The upper dotted line shows the H2S Uncertainty upper value (95% confidence). This is the upper uncertainty percentage represented as a concentration with a confidence level of 95%.
The lower dotted line shows the H2S Uncertainty lower value (95% confidence). This is the lower uncertainty percentage represented as a concentration with a confidence level of 95%.
The green line represents H2S data recorded after September 2023 displayed as monthly arithmetic means. This data was recorded using the updated calibration procedure. It has undergone the data ratification process and has been adjusted for drift and zero offset using H2S precision check data.
* The uncertainty values are based on an hourly average at a limit value of 100 parts per billion (ppb) and an analyser range of 2000 ppb and defined oxidiser efficiency; and are displayed as monthly arithmetic means.
We have shared the adjusted data with the UK Health Security Agency (UKHSA) which has reviewed its health risk assessments and produced a report based on the adjusted data which you can view here.
28 August 2024 Public Meeting
We held a public meeting on 28 August 2024 alongside UK Health Security Agency (UKHSA) to answer your questions on the adjusted data and UKHSA’s risk assessment. You can view the transcript of this meeting here.
If you wish to view the EA and UKHSA's presentations from the meeting please click on the links below.
We held a public meeting on 9 October 2023 which was attended by over 100 people. UKHSA and the director for health and care from Staffordshire County Council attended the meeting to answer any questions. Following the meeting we answered the remaining questions and posted them on our Engagement HQ page. You can view them here. You can also view the transcript from the meeting here.
