Background: Further evidence is needed on the effects that short- and long-term exposure to secondhand smoke (SHS) have on the respiratory health of patients with lung disease. Within the TackSHS project we aimed to assess the acute respiratory effects in lung function that result from short-term SHS exposure among patients with asthma and chronic obstructive pulmonary disease (COPD).Methods: The study design was an intervention trial with measurements before/after exposure to SHS in legal outdoor smoking areas. We studied patients with asthma or COPD from Czechia, Ireland, and Spain. Forced spirometry, peak flow and carbon monoxide (CO) measurements were performed pre- and 24 h post- exposure.Results: Overall, 60 patients were studied, 30 with asthma, and 30 with COPD; 35 (58.3%) were female. There were no significant differences observed in exhaled CO between pre- and 24 h post-exposure neither in women (p = 0.210), nor in men (p = 0.169).A statistically significant decrease in forced vital capacity (FVC) was seen, overall, in asthma participants (p = 0.02) and in forced expiratory volume in the first second (FEV1), (p = 0.02), FVC (p = 0.04) and peak expiratory flow rate (PEFR) (p = 0.04) in female asthmatic participants. The observed decreases in respiratory measurements in COPD were not significant. There were no reported increases in symptoms, respiratory medication, or use of health services 24 h after the exposure.Conclusion: We conclude that acute, short-term SHS exposure had a statistically significant effect on spirometry in female asthma patients but did not significantly modify spirometric indices 24 h later in COPD patients.