Respiratory disorders such as asthma and chronic obstructive pulmonary disease (COPD) are the fourth leading cause of death worldwide. While asthma is a reversible inflammatory disorder of the airways, which especially affects children, COPD is non-reversible and especially affects the elderly. The main causes of COPD are smoking in rich countries and indoor cooking smoke in poor countries. There is debate about causes of asthma, which almost doubled in some rich countries over the past 25 years.

In New Zealand, which is the context for this research, the burden of asthma and COPD is second only to cardiovascular diseases, in terms of disability-adjusted life years. New Zealand has one of the highest asthma rates in the world, with substantial ethnic inequality. Maori and Pacific children have higher prevalence and hospitalization rates for asthma than do children of other ethnic groups and the highest rate of increase in asthma over the last two decades was for Pacific Island children [ISAAC: International Study of Asthma and Allergies in Childhood]. The burden of respiratory illnesses for Maori and Pacific Islanders reflects higher smoking rates, poor housing with substantial overcrowding and possibly the unequal impacts of air pollution.

Immigrants are of special significance in the literature on asthma since migration involves exposure to a new set of pollutants and allergens and also affects diet and housing conditions, all which have possible links with asthma. This literature finds that the foreign-born arriving from countries with lower asthma prevalence experience a time-dependent increase in asthma symptoms. Comparisons between foreign-born and native-born survey respondents suffer from two potential problems: selection bias and reporting bias. Selection bias may mask the true extent of the migration-dependent increase in asthma if immigrants are more or less asthma-prone than non-migrants from their home country. Reporting bias may occur because of changes in access to health care with migration, with respondents only becoming aware of long-standing health disorders if visiting health care professionals after migrating.