A 10-year prospective cohort study finds that among 5,000 exposed workers and 5,000 unexposed workers, 200 and 100 cases of lung cancer develop, respectively. What is the Population Attributable Risk Percent (PAR%) if the prevalence of exposure in the total population is 30%?

• A Approximately 13.6%
• B Approximately 50%
• C Approximately 6.8%
• D Approximately 23.1% ✓

Explanation

Incidence in exposed = 200/5000 = 0.04; incidence in unexposed = 100/5000 = 0.02; RR = 2.0. Using Levin's formula: PAR% = Pe(RR-1) / [1 + Pe(RR-1)] × 100 = 0.30×(2-1) / [1 + 0.30×(2-1)] × 100 = 0.30/1.30 × 100 ≈ 23.1%. Cross-verification using incidence in total population: It = Pe×Ie + (1-Pe)×Iu = 0.3×0.04 + 0.7×0.02 = 0.012 + 0.014 = 0.026; PAR = 0.026 − 0.02 = 0.006; PAR% = 0.006/0.026 × 100 ≈ 23.1%. PAR% reflects the proportion of disease in the total population attributable to the exposure — even a modest RR of 2 combined with 30% population exposure yields a 23% population-level burden.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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