A review of a series of Australian studies on asthma and allergy identified synthetic bedding (using microfibre, nylon or acrylic materials), gas heating and passive smoking in homes as risk factors for asthma. Although environmental factors can trigger asthma in many, the relevant factors and the size of the effect varies widely in different countries and populations. The review analysed 56 studies that involved 137,840 people in Australia to find that in homes with these three triggers, people have exacerbated forms of asthma and had higher hospitalisation rates. Synthetic weave of pillows that can trap cat and dog allergens also triggers exacerbation in asthmatic children or elderly people.