People living with serious mental illness (SMI) die 10–20 years earlier than the general population, mostly due to preventable physical conditions such as cardiovascular or respiratory diseases. Yet, lung health remains largely overlooked in mental health care. 

In this prospective study, we followed a cohort of individuals with schizophrenia or bipolar disorder, all of them active smokers, for more than three years. We observed that half of the participants (51.7%) experienced a rapid decline in lung function—defined as a decrease of 40 mL or more per year in the forced expiratory volume in one second (FEV₁). This rate of deterioration is greater than that described both in the general population and in people with chronic obstructive pulmonary disease (COPD). 

Importantly, our data suggest that physical activity may play a protective role, particularly among those who already had lower lung function at baseline. Conversely, an increase in abdominal circumference was linked to a faster decline, and this effect appeared to vary with socioeconomic factors. 

This is the first longitudinal study to describe the course of lung function in people with SMI. The results highlight the need to integrate respiratory assessment into mental health care, as lung function decline is a powerful predictor of morbidity and mortality in other populations. Regular spirometry testing could provide a simple opportunity to identify those at higher risk and to guide preventive interventions. 

If confirmed in larger samples, these findings strengthen the case for incorporating lifestyle and physical health monitoring—particularly tobacco cessation, weight management, and physical activity promotion—into everyday psychiatric practice. Improving lung health should be seen as a central component of reducing the physical health gap for people with serious mental illness.