Assessment of smoking exposure by urine cotinine levels in severe COVID-19 patients: a case-control study

Coronavirus Disease 2019 (COVID-19), caused by SARS-CoV-2, has posed a significant global public health challenge, with long-term sequelae such as post-COVID-19 syndrome continuing to burden health systems. Tobacco use, a major preventable cause of morbidity and mortality, impairs the immune response and exacerbates respiratory diseases, including COVID-19. Passive smoking, an important but often overlooked public health problem, exposes non-smokers to harmful health risks and may contribute to worse outcomes in respiratory disease. This study aims to investigate the relationship between smoking exposure, including passive smoking, and the severity of COVID-19 using urinary cotinine levels to provide objective insights into a critical public health issue. This prospective observational study was conducted over six months in 2021 in the ICU of a tertiary care hospital. Forty-two patients were divided into four groups based on smoking and COVID-19 status: non-smoker/non-COVID-19, smoker/non-COVID-19, non-smoker/COVID-19, and smoker/COVID-19. COVID-19 diagnosis was confirmed by RT-PCR and/or imaging. Data collected included demographic information, clinical scores [APACHE II, mSOFA, GCS], respiratory function [stPaO2/FiO2 ratio], arterial blood gas parameters and routine laboratory findings. Urinary cotinine levels were measured by ELISA and normalised to spot urine creatinine levels to ensure accuracy. Urine cotinine levels were highest in the smoker/COVID-19 group and significantly higher than in the smoker/non-COVID-19 and non-smoker/COVID-19 groups (p = 0.010 and p = 0.002, respectively). Surprisingly, non-smoker/non-COVID-19 patients also had elevated cotinine levels, suggesting exposure to passive smoking. The stPaO2/FiO2 ratio was significantly lower in smoker/COVID-19 patients (p < 0.001), indicating impaired lung function. This study highlights the detrimental effects of smoking exposure, including passive smoking, on the severity of COVID-19. Elevated cotinine levels in non-smokers/non-COVID-19 patients emphasize the widespread public health impact of passive smoking and highlight the need for stronger policies and interventions to minimize tobacco exposure. Addressing both active and passive smoking is critical to improving public health outcomes and reducing the burden of respiratory diseases such as COVID-19. Further research is essential to validate these findings and to inform evidence-based public health strategies.