Asthma-COPD Overlap (ACO), also known as Asthma-COPD Overlap Syndrome (ACOS) is a chronic inflammatory, obstructive airway disease in which features of both asthma and COPD predominate. Asthma and COPD were once thought of as distinct entities, however in some, there are clinical features of both asthma and COPD with significant overlap in pathophysiology and symptom profile. It is unclear whether ACO is a separate disease entity or a clinical subtype of asthma and COPD. The pathogenesis of ACO is poorly understood, but it is thought to involve both type 2 inflammation (usually seen in asthma) as well as type 1 inflammation (seen in COPD). The incidence and prevalence of ACO are not well known. The risk factors for ACO are also incompletely understood, but tobacco smoke is known to be a major risk factor. Signs and Symptoms ACO presents with symptoms of both asthma and COPD. ACO presents in adulthood, usually after the age of 40 (after there has been significant tobacco smoke or other toxic fumes exposure), with symptoms of dyspnea (shortness of breath), exercise intolerance, mucous production, cough and episodes of symptomatic worsening known as exacerbations. Cause A history of significant and persistent noxious fumes exposure is required for the diagnosis of COPD, therefore is also required for the diagnosis of ACO. This can be due to tobacco smoking, indoor air pollution or outdoor air pollution. Pathophysiology ACOS presents with features of both asthma and COPD. Both asthma and COPD (as well as ACO) present with exacerbations, periods where symptoms deteriorate, with marked reductions in airflow. However, in asthma, the airflow limitation usually completely resolves after exacerbations, whereas in COPD it may not. ACO presents with a chronic airflow limitation or obstruction (due to inflammation), with characteristics of both asthma and COPD. Inflammation of the large and medium airways (classically seen with asthma) is seen in ACO. This consists of bronchoconstriction due to smooth muscle spasm as well as smooth muscle hyperresponsiveness (due to allergens or irritants) causing obstruction of airflow. Mucous production and inflammation in the airways can also cause airflow obstruction in asthma. Features of COPD (which includes the subtypes of chronic bronchitis and COPD) are also seen in ACO. These include the features of chronic bronchitis such as inflammation of the small airways and mucous production or hypersecretion. Peribronchial inflammation, which may lead to fibrosis (obliterative bronchiolitis). As well as features of emphysema including inflammation