• Asthma
• air pollution
• particulate matter
• ultrafine particles
• asthma epidemiology
• COPD epidemiology
• occupational lung disease
• paediatric asthma
• sleep apnoea

Danish researchers followed >57 000 older people over 10 years up to 2006.1 They made unique efforts to individually estimate exposure to traffic-related pollution for 35 years back in time, using modelled home–outdoor nitrogen dioxide (NO₂) concentrations. A 5.8 μg/m³ contrast in the 35-year average NO₂ concentration was associated with a 10% higher risk for a first-ever hospitalisation due to asthma. In other words, pollution of the last century appears to contribute to current hospitalisations. But does this interpretation make sense? First, I will present arguments against this conclusion. Second, I will explain how to make sense of this large and well-conducted study.

To clarify the interpretation of the results, it is worth formulating two basic hypotheses relevant to research on air pollution and chronic diseases like asthma where underlying chronic pathologies (eg, hyper-reactive airways) are superimposed by acute expressions of the chronic disease (eg, asthma attacks).2 Under this model, two primary hypotheses emerge for exposure to air pollution:

• H1: Exposure supports the development of the underlying chronic pathology and, thus, increases the pool of people with chronic conditions (chronic effects) prone to exacerbations or ‘events’.

• H2: Exposure triggers an acute event (or a state of frailty that results in an event with a delay of a few days or weeks) among those with the disease (acute and subacute effects).

In H2, the underlying cause of the asthma cases may (H1) or may not be air pollution. While mechanisms related to the two hypotheses are not necessarily independent, there are possibly different causes and pathways involved in the two time domains of effects. For example, exposure to cold air may trigger an asthma attack, while living in a colder climate may not necessarily relate to higher incidence of asthma.

H2 has been well studied for a range of events …