The major part of the approximately 150 cm 2 surface in the human nasal cavity is covered by respiratory epithelium, across which systemic drug absorption can be achieved. The olfactory epithelium is situated in the upper posterior part and covers approximately 10 cm 2 of the human nasal cavity. The nerve cells of the olfactory epithelium project into the olfactory bulb of the brain, which provides a direct connection between the brain and the external environment. The transfer of drugs to the brain from the blood circulation is normally hindered by the blood–brain barrier (BBB), which is virtually impermeable to passive diffusion of all but small, lipophilic substances. However, if drug substances can be transferred along the olfactory nerve cells, they can bypass the BBB and enter the brain directly.,  
40 mcg inhaled twice daily, approximately 12 hours apart, is the recommended starting dose. For patients who do not respond adequately to 40 mcg after 2 weeks of therapy, increasing the dosage to 80 mcg twice daily may provide additional asthma control. The maximum recommended dosage is 80 mcg twice daily. The starting dosage is based on the severity of asthma, including consideration of the patients’ current control of asthma symptoms and risk of future exacerbation. Improvement in asthma symptoms can occur within 24 hours of the beginning of treatment and should be expected within the first or second week, but maximum benefit should not be expected until 3 to 4 weeks of therapy. Improvement in pulmonary function is usually apparent within 1 to 4 weeks after the start of therapy. The National Asthma Education and Prevention Program Expert Panel defines low dose therapy as 80 to 160 mcg/day, medium dose as 161 to 320 mcg/day, and high dose therapy as more than 320 mcg/day for children ages 5 to 11 years. The Global Initiative for Asthma (GINA) guidelines define low dose therapy as 100 mcg/day in this age group. Titrate to the lowest effective dose once asthma stability is achieved.