Of course, radon is in the natural outdoor air as well, but it is diluted by all the air that is available. The Environmental Protection Agency (EPA) did a national survey in the United States and determine that the average indoor air concentration ( picocuries of radon per liter of air) is about three times higher than the average outdoor air concentration ( picocuries per liter). The average indoor air concentration is three times less than the recommended limit ( picocuries per liter). Even though, on average, we could expect our house to have about picocuries per liter of radon, the EPA estimates that nearly 1 out of every 15 homes in the United States has elevated radon levels.
The serendipitous discovery and subsequent clinical success of penicillin prompted a large-scale search for other environmental microorganisms that might produce anti-infective natural products. Soil and water samples were collected from all over the world, leading to the discovery of streptomycin (derived from Streptomyces griseus ), and the realization that bacteria , not just fungi, represent an important source of pharmacologically active natural products.  This, in turn, led to the development of an impressive arsenal of antibacterial and antifungal agents including amphotericin B , chloramphenicol , daptomycin and tetracycline (from Streptomyces spp. ),  the polymyxins (from Paenibacillus polymyxa ),  and the rifamycins (from Amycolatopsis rifamycinica ).