Asthma is a common chronic health condition, affecting 5% of the United States adult population. In most developed countries, the prevalence of asthma and its severity continues to increase. Understanding the factors contributing to asthma morbidity and mortality has important clinical and public health implications. This article evaluates the evidence that secondhand smoke exposure is a risk factor for new-onset asthma among adults and exacerbates pre-existing adult asthma, resulting in greater symptom burden and morbidity.