Sometimes more than one risk factor will be linked to a particular disease. In theory, this requires estimating joint relative risks and exposure distributions. Under certain circumstances, however, a simple method can be used to calculate a combined PAF that involves multiple risk factors (Ezzati and others 2004):

PAF^combined = 1 - ∏(1-PAFⁱ)                                                                           

where

PAFⁱ is the PAF for risk factor i

The logic here is as follows. 1-PAFⁱ represents the proportion of the disease that is not attributable to risk factor i. Multiplying these risks yields the share of the disease that is not attributable to any of the risk factors, and subtracting this from 1 leaves the share of the disease that is attributable to the set of risk factors considered.

Say that we have 2 risk factors: ^[1]

PAF^combined = 1 - (1-PAF¹)(1-PAF²)

 Following from the discussion above, the combined adjustment factor can be calculated as:

 ((1-PAF^combined _Distal) / (1-PAF^combined _Full)) = [(1-PAF¹ _Distal)(1-PAF² _Distal)] / [(1-PAF¹ _Full)(1-PAF² _Full)]

 = [(1-PAF¹ _Distal)/(1-PAF¹ _Full)] * [(1-PAF² _Distal)/(1-PAF² _Full)]

 = [∑RR¹(x)P¹ _Full(x)/∑RR¹(x)P¹ _Distal(x)] * [∑RR²(x)P² _Full(x)/∑RR²(x)P² _Distal(x)]        

 In other words, the combined adjustment factor is a simple multiplication of the individual adjustment factors.