1. Legal approach: Threshold limit value for a single gas (TLV)
2 . Negotiation: pragmatically improving the limit values from year to year by regional negotiations
3. Mathematics: Limits for weighted mean values summing up the effect of various gases
4. International negotiations: Prescription of annual decrease by x% points of national emissions
___________________________________________________
Sources:
- https://en.wikipedia.org/wiki/Threshold_limit_value
https://www.researchgate.net/publication/263707803_Gasoline_Risk_Management_A_Compendium_of_Regulations_Standards_and_Industry_Practices/figures?lo=1 - https://www.who.int/phe/delivering-air-quality.pdf
- https://www.eea.europa.eu/data-and-maps/figures/air-quality-standards-under-the-1
- https://www.eea.europa.eu/data-and-maps/indicators/greenhouse-gas-emission-trends-7/assessment