On 7 December 1972 the crew of the spaceship Apollo 17 took a color photograph of the Earth from 18,000 miles out in space. The ‘Blue Marble’ image they sent back not only raised public awareness that most of the planet’s surface is covered by water, but reminded us of the deep connection humans have with our seas and oceans. Close scrutiny of this and other images of the Earth from space reveal a human preference for settlements close to the sea coast and vividly illustrate the worldwide use of the oceans for transport and trade. Dependence on marine resources is clear from the ceaseless activity of the myriad fishing fleets and container ships roaming both coastal and open waters, and in the hundreds of towering oil rigs endlessly feeding off undersea petroleum reserves.

Back on terra firma, human connections with the sea are even plainer to see. From seaside holiday resorts and marinas to major ports and industrial complexes, over 370,000 miles (592,000 km) of the coastline of the continents are adorned with our artefacts. Over a third of the human population (ca. 2.5 billion people) now lives within 60 miles (96 km) of the coast, with many thousands more joining them every year. In this essay, ways of living sustainably and in harmony with marine ecosystems are discussed.

Although coastal areas are deemed desirable locations in which to reside or visit, we often fail to grasp the extent of risks to our health and well-being. For example, globally, there were ca. 1,562 flood disasters from 1994 to 2004 that killed ca. 120,000 people and affected two million more overall. Most of these events were played out in low elevation coastal zones. One hundred and eighty countries have populations located in such areas, mainly in large urban settlements. Twenty-one of the world’s 33 megacities (> 8 million people) are coastal. When extreme events hit, the lives of enormous numbers of people are put in jeopardy and demands on healthcare systems escalate sharply. When Hurricane Mitch hit Honduras in October 1998, it caused 18,000 deaths, mainly in the coastal zone. Similarly, in the United States, when Hurricane Katrina made landfall in New Orleans in August 2005, 1,800 deaths ensued. The level of economic development of a community tends to modulate the severity of impacts. When Hurricane Sandy hit wealthy New York in 2012, 285 people died, while Typhoon Haiyan striking the less prosperous Philippines in November 2008, killed 5,200 people. By the 2000s, millions more people will face storm surges and floods every year as sea level rises. As if to confirm this trend, at the time of writing (summer 2017), Hurricane Irma has passed through the Caribbean and Florida causing utter devastation. The costs in terms of human lives and economic damage await quantification, but will undoubtedly be enormous.

Coastal dwellers are also vulnerable to undersea seismic events. The Asian (Indonesian) earthquake and tsunami of December 2004 killed at least 226,000 people in coastal areas of 13 countries, with over 500,000 people injured. A further ca. 150,000 people died from infectious diseases following the disaster. Up to five million coastal residents lost their homes, or access to food and water and around one million people were left without a means of making a living. The collapse of sewage treatment and land drainage systems resulted in the discharge of pathogens into coastal waters. The public (both locals and visitors) were then exposed to dangerous viruses and bacteria through skin contact and following the consumption of contaminated fish and shellfish. On a daily basis, wastewater and human sewage of a huge portion of the world’s population (> 640 million people) are currently discharged daily, directly or indirectly, into coastal seas, mostly without treatment. Past research indicates that this generates ca. 120 million cases of gastrointestinal diseases and more than 50 million cases annually of respiratory disease. Recent data are not available so it is hard to assess current trends. However, we do know that viral and to a lesser extent, bacterial and protozoan pathogens in polluted coastal waters contaminate edible shellfish and fish resulting in ca. four million cases of hepatitis A and E, which cause ca. 40,000 deaths, together with a further ca. 40,000 episodes of disease that lead to long term disability. As seawater temperatures rise with global warming, rates of pathogen infections will increase and their geographic range will extend. Added to this, an estimated 3–5 million cases of cholera result each year in 100,000–120,000 deaths. Harmful algal blooms are also increasing in frequency. Around 60,000 cases annually of gastroenteritis and respiratory irritation are due to consumption of seafood containing algal toxins. Only a small proportion are diagnosed, usually as paralytic shellfish poisoning, amnesic shellfish poisoning, and Ciguatera poisoning.

For many coastal inhabitants in temperate, higher income countries where extreme storms, seismic events, and sewage pollution are less common, the threats mentioned above may seem remote. Nonetheless, those living and working at sea or along coastlines are exposed everywhere to many other hazards. Of note, ca. 372,000 people globally drown each year. Those in fisheries and the offshore oil industry are especially at risk. From 2000 to 2006, the average annual fatality rate in the United States was 115 deaths per 100,000 fishermen. This is three times greater than that of the next most dangerous occupation and more than 25 times that of the US national average for all workers. In offshore oil and gas workers, death rates were 27 per 100,000 in the United States between 2003 and 2010, seven times greater than the national average.

Recreation in coastal areas generates a surprising number of health issues. Worldwide, more than 355,000 people are injured annually in recreational boating accidents, with more than 40 percent of injuries requiring medical treatment beyond simple first aid. Shallow water and open sea recreation brings people into contact with poisonous or aggressive marine animals in many locations. Dangerous species encountered include sharks, barracuda, the blue ringed octopus, lionfish, stingrays, cone shells, and swarms of jellyfish. Despite wide public interest and concern, deaths and injuries due to these creatures are still quite rare.

A less obvious, long-term health threat arising in the sea involves chemical pollutants contaminating seafood. This is perhaps the most underestimated, insidious danger of all those discussed so far. Persistent pollutants such as heavy metals (especially mercury), polyaromatic hydrocarbons, and a wide range of synthetic organic compounds (PCBs, Bisphenol A, brominated flame retardants, dioxins, perfluorinated compounds, etc.) have been linked to clinical disorders including neuropathy, cardiomyopathy, endocrine disruption, vascular disease, and cancers. Body burdens of environmental chemicals are associated with the changing incidence of a wide variety of illnesses worldwide, so much so that they may be influencing global patterns of disease.

This litany of ocean-related health threats is perhaps both surprising and alarming, and clearly warrants much greater attention as ever higher numbers of people move to the coast. However, it is important to consider, with equal diligence, the enormous range of opportunities and health benefits offered by our seas and coastal areas, many of which have previously been under-exploited or simply overlooked. Good health and well-being are intimately linked with economic prosperity and education. In many of the world’s poorest countries, marine ecosystems generate employment for millions of people and offer an attractive environment in which to live. Capture fisheries and aquaculture provide jobs for 10 to 12 percent of the world’s population. Over two billion people work in the marine energy sector (non-renewable and renewable), mining industries (aggregates and deep sea mining), maritime transport, and tourism. In total, approximately two thirds of the global gross national product (GNP) is generated within 100 km of the sea coast, with over nine billion tonnes of goods being shipped around the world by sea each year. Shipping capacity will more than double by 2030. Many other marine-related jobs contribute to sustaining coastal communities; for example, employment in the extraction of salt, processing and preserving of fish and shellfish, manufacturing refined petroleum products; the building, repairing and maintaining ships, boats and floating structures; coastal engineering; and the construction of flood defenses, ports, coastal towns and cities. All of these activities help people to live healthy fulfilling lives.

Marine and medical scientists are keen to point out the dietary health merits of seafood. Fisheries produce ca. 160 million tons of fish per annum that represent ca. 16 percent of animal protein consumed globally. Fish and shellfish rich in omega-3 fatty acids, when combined in a ‘Mediterranean diet’, consistently lead to health and well-being improvements, conferring cardio-protective effects and a reduced incidence of mammary and prostate cancers. Benefits for neurological function also seem likely.

Other marine organisms have facilitated the discovery of new pharmaceuticals. For example, Ecteinascidin 743 is a potent anti-cancer drug derived from the Caribbean sea squirt; a series of powerful chemotherapy drugs, the dolastatins, have been extracted from sea hares, Dolabella auricularia; and Conotoxin, a potent anti-pain drug, is extracted from marine cone shells. Brevenal from the Florida red tide alga also shows promise in treating cystic fibrosis.

Equally exciting is the renewed interest is the use of our seas and oceans to directly foster improvements in health and well-being. In the eighteenth century, seawater was considered helpful in treating ‘glandular’ diseases such as scurvy, scrofula, jaundice, and tuberculosis. One of the first seawater health spas, created in Brighton, England, came to prominence when attended by the Royal family, leading to other coastal hospitals being established, including the Royal Seabathing Infirmary at Margate in 1791. These institutions offered so-called ‘thalassotherapy’ to help the weak and sickly recuperate. With progress in medical science, especially the development of pharmaceuticals, interest moved on from thalassotherapy until around ten years ago when the ‘blue gym’ concept emerged. This involves a more scientific approach to assessing health and well-being benefits of spending time in and around outdoor blue spaces, such as seashores, coastal paths, and estuaries. In England, an analysis of census data for 48 million people showed that they reported better health if they live at the coast. Effects were especially strong in areas of deprivation, suggesting that coastal living could be used to help tackle health inequalities. Physical activity promotes mental and physiological health so if coastal environments encourage outdoor pursuits, potential public health benefits could be enormous, including reducing the risk of obesity and of developing diabetes, cardiovascular and respiratory diseases as well as various cancers. Further health pathways may be via stress reduction and restoration of mental well-being, and direct physiological effects mediated through exposure to biologically active molecules present in sea spray. The contention is that these biochemicals, once inhaled, reduce inflammatory responses and foster better health.

In summary, coastal dwellers represent a growing proportion of the global population at a time when threats to health and well-being from the marine environment are increasing rapidly. More frequent extreme weather events, sea level rise, rising sea temperatures, and ocean acidification associated with climate change are exacerbating dangers posed by harmful algal blooms, microbial pathogens, and chemical pollution. Paradoxically, as we have seen, this is at a time of growing recognition of the health and well-being benefits of coastal living. The medical community shows little awareness of the complex relationship between marine-related threats and opportunities and their relevance to medical practice. Illnesses arising from contact with the sea or sea foods tend to be regarded as being of minor importance compared to health risks associated with, for example, smoking, alcohol consumption, obesity, and other lifestyle factors. This may be true in a day-to-day sense, but over the longer term, developing better ways of interacting with the sea and of living in coastal environments may greatly improve overall public health.

It is therefore timely to re-evaluate the topic of ‘oceans and human health’ in a global context.¹ This will put us in a position to mitigate, adapt to, or avoid threats and relieve ourselves of the sense of gloom and foreboding which marine pollution, climate change, and extreme events generate. By fully recognizing that our seas and oceans can play a role in maintaining or improving health and well-being, we can set course for public health policy interventions that help us to significantly lower the global burden of disease.