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Predicted environmental concentration is the process contribution, from emission rate times a dispersion factor, added to the existing background. Screening criteria commonly treat a process contribution below 1% of the long-term standard as insignificant, and compliance is judged on the total against the standard.
Predicted concentration
Total = background + emission rate x dispersion factor
Significance
Process contribution as a percentage of the air quality standard
Air quality assessment for permitting must follow the regulator's approved methodology with site-specific modelling and monitored background data.
Because the standard applies to the total concentration a person breathes. A small process contribution can still cause exceedance where background is already close to the limit.
From a dispersion model run for the site, or from a generic screening factor. Site-specific modelling with local meteorology is required for permit determinations.