A team of epidemiologists is interested in assessing the relationship between long-term exposure to a newly identified industrial solvent (Solvent Z) and the development of interstitial lung disease (ILD). They enroll 800 factory workers without ILD at baseline and classify them into two groups based on documented workplace exposure: 400 workers with high-level exposure to Solvent Z and 400 workers with minimal or no exposure. Over the next 12 years, they perform annual clinical evaluations and imaging studies to document new cases of ILD. At the end of the follow-up period, 56 of the exposed workers and 16 of the unexposed workers have developed ILD. Which measure of association should the investigators calculate to most directly compare the risk of ILD in exposed versus unexposed workers in this cohort study design?