1. Combined effects of smoking and dust on the risk of FEV1 falling below a specified threshold.
Combined effects of smoking and dust on the risk of FEV1 falling below a specified threshold. It is assumed that FEV1 is normally distributed in the population and that a given cumulative exposure to dust shifts the distribution downwards by a decrement “d”. In the absence of smoking (A) the relative risk of having an FEV1 below a threshold value as a consequence of this dust exposure is given by the ratio of areas under the curves (P0 + P1)/P0. If it is assumed that a given cumulative exposure to tobacco smoke causes an additional downward shift in the distribution of FEV1 by a decrement “s”, and that the effects of dust and smoking on FEV1 are additive, the position will be as depicted in B. The relative risk of having an FEV1 below the threshold is again given by (P0 + P1)/P0. As age increases, cumulative exposure to tobacco smoke among smokers also increases and so too does the decrement “s” attributable to smoking. The effect is a reduction in the ratio (P0 + P1)/P0. Thus, the relative risk of having an FEV1 below the threshold as a consequence of a given cumulative dust exposure would be expected to decline with age in smokers.
David Coggon, and Anthony Newman Taylor Thorax 1998;53:
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