Biological sensitivity to chemicals at levels of parts per billion
There are no normal factors in our lives where we can easily understand numbers as low as one in a billion, so I need some everyday examples. One-in-a-billion sensitivity is equivalent to detecting one individual grain of salt in a kilo of salt. It sounds extreme, but in social terms there have been, and are, politicians and religious leaders who have significant influence over more than a billion other people. The major religions are invariably based on the teachings of a single prophet, and the statements from religious or political leaders, from the Pope to the leaders of India, or China, all impact the lives of a billion other humans. So the importance of one in a billion is not that rare, even if we think it is a little extreme.
Effects resulting from parts per billion may sound incredible, but many animals respond to the presence of impurities at these levels. Since we have learnt of many important examples, it suggests that our sensitivity may equally exist in other circumstances. I am sure many effects will continue to emerge with future studies as our measurement technology improves. One positive example is seen in the way pheromones from female moths are detected. The male moth will sense the presence of a female moth when the concentration of her pheromones in the air is as little as one part per billion. By heading up the concentration gradient, it is then able to chase after her; scent detection can be effective from a kilometre away. This is not exceptional: the homing instincts of salmon and other fish, turtles, etc. that return to specific locations to lay their eggs, rely on an equally extreme sensitivity to scents and trace impurities in the water.
A second animal example is dogs. Their sense of smell is incredibly superior to ours: they can detect other dogs, cats, or humans over distances of 100 metres. Because their scent skills are very precise, we use them in tracking (as in manhunts), to detect drugs and explosives (e.g. in airports), and even in medicine—there are many examples where they have been trained to detect cancers, diabetes, and epilepsy from the chemical emissions given out by the patients. Such sensitivity is currently far beyond our technological attempts to achieve these diagnoses or to make sensors with the same discrimination.
Compared to dogs, humans have a far inferior sense of smell, which is further weakened by the technologies that have driven us into cities and high-population clusters. Those who enjoy a truly rural lifestyle can sense far more subtle perfumes and odours than city dwellers. The same is true of hearing and of other animals. The main observation of ornithologists who study bird song is that, although there are some dialect differences between city and country birds of a particular species, city birds have to sing louder—often double the volume compared with country birds. So for all creatures, including humans, the technological benefits of city life have caused a considerable loss in natural skills, including sensitivity to sounds, sights, and smells.
Scientific techniques, benefiting from highly sophisticated analyses of tiny concentrations of background materials, are increasingly feasible and useful in many unexpected ways. For example, to monitor trends in drug usage in northern Italy, measurements have been made of drug concentrations in the Po river. Despite sewage treatments, traces of drugs survive, and so changes in the background level reflect changes in drug usage in the region. Analysis also reveals the level of drugs ingested by non-users merely as a consequence of using tap water.