To investigate the effect of reducing the level of allergens and pollutants in the bedroom and living room by placing a "Dyson air purifier", on poorly controlled asthmatic subjects.

Asthma is one of the most common chronic diseases. Little change in morbidity and mortality has occurred despite improvements in pharmacotherapy. In the last few decades, there has been an increase in the prevalence of asthma and other allergic diseases. The precise cause for this increase in disease prevalence is not known but it has coincided with changes to the quality of indoor air with increases in the levels of allergens and pollutants. Bedroom exposure to dust-mite allergens has been linked to worsening asthma symptoms and increase in bronchial responsiveness. In places where dust mites cannot thrive, allergens from cat, cockroach and Alternaria assume importance. High indoor temperatures and humidity may, by a number of mechanisms, increase the allergenic burden, particularly the proliferation of house-dust mites and moulds. Therefore, modern living conditions are associated with a higher risk of allergen exposure causing increase in sensitisation and symptoms of asthma. In addition to allergens, the indoor environment contains other biological materials (such as microbiome and endotoxin), and pollutants (gases and particulate matter) which can adversely affect asthma development and morbidity. Indoor pollutants include smoke from cigarettes and wood, coal or gas fires, particulate materials associated with bio-fuel combustion, chemical vapours and gases including nitrogen dioxide (NO2), formaldehyde and volatile organic compounds (VOCs). The latter may come from sources including building products, cleaning agents, and paints. One such VOC is formaldehyde, which can be irritant to both upper and lower respiratory tract. Small particulate matter (PM2.5) is particularly damaging as it gets to the small airways of the lung. Major indoor sources of NO2 and particulate matter include gas stoves and cigarette smoke but outdoor sources such as traffic and industrial pollution can also contaminate indoor environment.

It has also been suggested that exposure to pollutants can potentiate the effects of allergen. Indeed, a combination of high levels of indoor pollution and allergens is causally related to the development and severity of asthma. Allergens, microbiome and pollutants can interact with each other to augment the immune response leading to harmful effects on the airways.

Thus, indoor air pollution is a significant environmental trigger for acute exacerbation of asthma (and other respiratory conditions such as COPD), leading to increasing symptoms, emergency department visits, hospital admissions and even mortality. An estimated 75% of hospital admissions for asthma are avoidable. Maintaining high air quality with lower levels of allergens and pollutants is therefore important in improving the health of individuals with asthma and other respiratory diseases. Therefore, a feasible and practical intervention that can reduce allergen and pollutant levels in the indoor air should reduce morbidity and improve asthma control.

Inclusion Criteria:

• Patients between 18 to 75 years of age with a confirmed diagnosis of mild persistent to moderate persistent asthma (BTS steps "regular preventer therapy" to "additional add-on therapies")
• ACQ6 score >1.5.

Exclusion Criteria:

• Patients with significant chronic respiratory disease such as COPD or bronchiectasis.
• Patients with any severe disease (such as cardiovascular disease, dementia etc.), where adherence to the study protocol may cause an unjustified stress.
• Those who are being treated with allergen specific immunotherapy.
• Patients with a history of significant alcohol or drug abuse.
• Patients who are taking an investigational drug for asthma.
• Patients who are unwilling, unlikely or unable to comply with the study protocol, as assessed by the study team members.
• Patients who are likely to be started on biological therapies for asthma (omalizumab, mepolizumab, reslizumab, dupilumab) during the study period.
• Pregnancy.
• Patients already using air purifiers in their dwellings.
• Patients planning to move house during the study period.