Rosemary oil has antibacterial properties that can treat a host of your respiratory problems. The scent of rosemary oil eases throat congestion, and treats allergies, sore throat, cold and flu. Its antispasmodic qualities it is also beneficial in treating bronchitis and asthma. 
In the book ‘Healing Foods’, it is said that the strong antioxidant action of rosemary oil can help reduce inflammation, thus helping to lower the risk of asthma.