Wood heaters are the main source of air pollution in many Australian cities and towns in winter. Australians with asthma often suffer from worsening of their symptoms from breathing air polluted with smoke from wood heaters. Portable air cleaners with high-efficiency particulate air (HEPA) filters are a promising way for reducing smoke pollution in homes throughout the year. However, their ability to improve asthma control and related quality of life has not been tested in Australia. This study will evaluate the ability of portable air purifiers with HEPA filters to: (a) Reduce air pollution in homes with and without wood heaters; (b) Reduce asthma symptoms and improve quality of life for people with asthma in four Australian regions that are affected by wood heater smoke during winter.