I smoke a cigarette just after workout at gym. Can that be more dangerous than smoking at a more ordinary time? For example, can it cause sudden cardiac arrest?
Smoking in general is unhealthy. Smoking can result in emphysema, COPD, pulmonary hypertension and other ugly diseases [1,2,3]. After a workout your heart and respiration rates are higher than at basal levels. Although I do not have data to back me on this, based on scientific evidence on cardiopulmonary effects  after a workout and the hazards of smoking [1,2,3], I would deduce you would have higher nicotine intake, as well as other terrible chemicals that are in cigarettes. I am not sure if cardiac arrest would be a primary concern with smoking but rather a secondary event to pulmonary hypertension [2,3]. In short I would seek advice and support on quitting smoking.