Vascular plants, mosses and lichens react sensitively to the exposure to air pollutants and pollution from ammonia (NH₃). Critical concentration values for ammonia to protect vegetation (so-called critical levels) were defined for risk assessment in the Geneva Convention on Long-Range Transboundary Air Pollution (CLRTAP). The project was used to review the current scientific status of those impact thresholds. In addition, extensive experimental studies were carried out to test the sensitivity of numerous native vascular plant species to NH₃. The focus here was on species protected under the Natura2000 legislation and common in habitats in Germany.