The Government Says

Monitoring air quality and dust on the HS2 Phase One route (April 2026)

Monitoring air quality and dust on the HS2 Phase One route (April 2026)
Local copy of BC_AirQuality_Monthly_Report_April_2026.pdf

https://www.gov.uk/government/publications/monitoring-air-quality-and-dust-on-the-hs2-phase-one-route-april-2026

seen at 11:32, 29 June in Policy papers and consultations.