Asthma can be divided into:

extrinsic - implying a definite external cause

intrinsic or cryptogenic - when no causative agent can be identified.

Extrinsic asthma occurs most frequently in atopic individuals who show positive skin-prick reactions to common inhalant allergens. Positive skin-prick tests to inhalant allergens are shown in 90% of children and 50% of adults with persistent asthma. Childhood asthma is often accompanied by eczema. An overlooked cause of late-onset asthma in adults is sensitization to chemicals or biological products in the workplace.

Intrinsic asthma often starts in middle age ('late onset'). Nevertheless, many patients with adult-onset asthma show positive skin tests and on close questioning give a history of respiratory symptoms compatible with childhood asthma.

Non-atopic individuals may develop asthma in middle age from extrinsic causes such as sensitization to occupational agents or aspirin intolerance, or because they were given β-adrenoceptor-blocking agents for concurrent hypertension or angina. Extrinsic causes must be considered in all cases of asthma and, where possible, avoided.