This document provides a clear, reproducible account of how PM₂.₅ data for Hertfordshire and Bedfordshire was collected, cleaned, and analysed. It supports the findings presented in the Particulate Matter (PM₂.₅) in Ambient Air 2025 report and demonstrates the workflow used to generate the figures included there. The focus here is on methodology rather than interpretation. By documenting each step — from data import to quality assurance and visualisation — this R Markdown file aims to make the analytical process open, repeatable, and accessible for others working in air‑quality analysis or reproducible research. The analysis predominantly uses open‑source tools in R, ensuring that all steps — from data import to visualisation — can be independently reviewed and repeated. Where this has not been done an explanation as to why is provided.

This analysis examines severe weather events in the United States from 1990 to 2011 using NOAA’s Storm Events database, focusing on fatalities, injuries, property damage, and crop damage.