Editor’s Note: This article was originally published in 2018 and is preserved for historical interest. Some recommendations, regulations, and scientific evidence have changed since it was written. It reflects the research and perspectives available at that time.

According to a study published in the British Medical Journal (July 8, 2000;321:88-92) the incidence of asthma is up by 250% in the last two decades. Prescriptions for steroid medications for asthmatics has increased by 600% in the last 20 years. Hay fever is also on the rise. Among non-smokers, the rate of hay fever increased from 6% to 20%.