Sweet Basil relieves Asthma?

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I have a friend whose wife is prone to severe asthma attacks. He told me that since he surrounded his house with Sweet Basil (Ocimum basilicum), the number of attacks has dramatically gone down. According to him, normalcy in breathing is also restored faster when his wife inhales the fragrance of crushed Sweet Basil in her hands. What could be in Sweet Basil that curtails asthma attacks?