Treating patients with chronic obstructive pulmonary disease (COPD) with high concentration oxygen therapy can cause hypercapnia (increased blood carbon dioxide) and acidosis (increased blood acid content). These are associated with worse clinical outcomes. High flow oxygen is often used to deliver inhaled medication using nebulisers. This can cause harm. An alternative to oxygen driven nebulisers are air driven nebulisers, which might reduce the risk of hypercapnia and acidosis. We propose a randomised controlled trial to compare the effects of air and oxygen driven nebulisers on carbon dioxide levels and acidosis in patients admitted with COPD to Wellington Regional Hospital and Hutt Valley Hospital. The findings of this study will provide evidence for the safest treatment for patients with COPD and whether health services should change from use of oxygen driven nebuliser to air driven nebuliser delivery.