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Caffeine: Emerging contaminant of global rivers and coastal waters

  • Caffeine is the most consumed psychostimulant in the world, and a regular part of many daily lives, whether contained in coffee, chocolate, energy drinks, or pharmaceuticals.
  • Partially excreted in urine, it is now ubiquitous in rivers and coastal waters. So much so that its detection is used to trace wastewater and sewage pollution. A new study found it to be in more than 50% of 1,052 sampling sites on 258 rivers around the globe. Another new study enumerates caffeine harm in coastal and marine environments.
  • This continual flow of caffeine into aquatic ecosystems is causing concern among scientists due to its already identified impacts on a wide range of aquatic life including microalgae, corals, bivalves, sponges, marine worms, and fish. Most environmental impacts — especially wider effects within ecosystems — have not been studied.
  • Soaring global use of products containing caffeine means the problem will worsen with time. Untreated sewage is a major source. And while some sewage treatment facilities can remove caffeine, many currently can’t. Far more study is needed to determine the full scope and biological impacts of the problem.

Whether it’s found in a cup of hot joe, a chunk of chocolate, or an energy drink, caffeine is one of our favorite tipples — but alas, all the mild psychostimulant contained in those daily “pick me ups” has to go somewhere.

A just published global study of pharmaceutical pollution sampled 1,052 sites in 258 rivers — covering 104 countries across multiple continents. Researchers found that freshwater caffeine pollution is rife the world over: It was among the ingredients with the highest concentrations in river surface water and detected in more than 50% of sampling sites.

This revelation comes in the wake of another recent study, published in the journal Chemosphere, indicating that the global consumption of caffeine — including in food and beverages and in common pharmaceuticals — is an emerging concern for coastal and marine environments.

A team of researchers, led by Rosa Freitas at the University of Aveiro in Portugal, reviewed the current body of evidence and noted caffeine impacts on a wide range of marine species. Exposure, they said, can have harmful effects on aquatic creatures even at low concentrations.

These two studies taken together spotlight caffeine pollution in both fresh and saltwater ecosystems as a serious global issue.

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A ubiquitous substance

Caffeine is a habitual part of many millions of lives, and not surprisingly it’s the most widely consumed psychostimulant in the world. Humanity’s daily fix typically starts its journey into waterways at the coffee machine, in a soft drink can, candy wrapper, or medicine cabinet.

Much of it is metabolized and very little is excreted, with only around 5% of consumed caffeine thought to pass out of the body via urine — but those tiny amounts add up. More enters water systems when leftover coffee, tea, or soft drinks get tossed, coffee grounds are dumped down the sink, or medications get thrown away.

Once this caffeine burden reaches treatment plants — in places where they exist — systems vary in their ability to remove caffeine. Between 64 and 100 percent is removed by secondary and tertiary level wastewater treatment plants (WWTPs), according to one estimate. Primary wastewater treatment can be far less effective. A study in Barbados found that such treatment had a removal efficiency of only around 38%.

“Although the WWTPs removal capacity is high, the constant release of such substances into the environment may result in high concentrations in some aquatic systems, such as freshwater bodies,” says Freitas. However, the main problem is the release of caffein in untreated wastewater.

Coral bleaching in American Samoa. Studies suggest that caffeine can stress coral algae, potentially contributing to bleaching. Image courtesy of The Ocean Agency/ Ocean Image Bank.

According to some estimates, up to 80% of global wastewater flows into the environment without treatment. That’s because treatment is simply not available to all, and where it isn’t, waste often floods freely into water sources. In addition, hundreds of modern cities with aging sewage treatment systems — including New York, London and Paris — are burdened with Combined Sewer Overflows (CSOs). In these antiquated systems, wastewater and stormwater travel through the same pipes to treatment plants. This system works fine until heavy rainfalls overwhelm treatment facilities, forcing the shunting of raw sewage into streams and estuaries, releasing a toxic cocktail of pollutants including caffeine.

It is our daily coffee fix that is considered the major culprit behind much caffeine pollution, as consumption and concentrations are so high, particularly in developed countries. According to the National Coffee Association USA, Americans consume 656 million cups of coffee per day, equating to 2 cups per person daily. But European consumption peaks even higher, with EU countries gulping down around one-third of the world’s global coffee production — consuming around 3,244 million tons in 2020/21. Expectations are that the global coffee market will only continue growing, particularly in the Asia-Pacific region.

Coffee growing and processing are also implicated in caffeine pollution. Producing our daily brew requires a large amount of water, which when disposed of has environmental consequences, such as eutrophication. Coffee processing in Costa Rica has been linked to one of the highest concentrations of caffeine ever recorded in surface water.

Vacations too add to caffeine contamination peaks: “The seasonal increase in population due to tourism into the coastal areas can… play an important role,” notes Freitas’ study, with upticks in caffeinated beverage use linked to higher contamination levels in waters at tourist hotspots. Take for example, Lake Balaton, Hungary, one of Central Europe’s largest lakes. Effluent into the lake, including caffeine, was shown to peak in summer months, when thousands of visitors descend on the region. That peak poses a “high ecological risk for freshwater ecosystems.”

Columbia Boulevard Wastewater Treatment Plant in Portland, Oregon, United States. Modern plants generally remove high levels of caffeine from wastewater, but many aging facilities fail to do so. Also, antiquated CSOs in hundreds of cities release pulses of raw sewage during intense storms, events that are becoming more common as climate change escalates. Image by Eli Duke via Flickr (CC BY-SA 2.0).

Concern about caffeine

Too much caffeine is known to carry health risks for human beings, including nervousness, nausea, increased heart rate, and other side effects. As the study published in Chemosphere indicates, aquatic species are also at risk.

Freitas’ team investigated the long-term impact of caffeine exposure on polychaetes, or marine worms. They found that caffeine can delay the regeneration of worm segments in one species, Diopatra neapolitana. “Such negative impacts impair the animal’s physiological and biochemical performance, such as the capacity to burrow and hide from predators,” says Freitas. In addition, the increased energy required to accomplish segment regrowth could have knock-on effects for the worm’s reproductive capacity.

Caffeine has also been found in microalgae, coral reefs, bivalves and fish, the study notes, due to bioaccumulation, with various repercussions: “[C]affeine residues had been demonstrated to have adverse impacts on aquatic organisms, at environmentally realistic concentrations, inducing oxidative stress and lipid peroxidation, neurotoxicity, changing energy reserves and metabolic activity, affecting reproduction and development and, in some cases, causing mortality.”

Examples of specific species impacts include the Fathead minnow (Pimephales promelas), which can suffer inhibited growth, and purple sea urchin (Paracentrotus lividu), which can see impaired reproduction due to caffeine exposure. Caffeine may also exacerbate the stress effects of ocean acidification and higher temperatures on coral algae, potentially contributing to global bleaching events.

“Although caffeine has a relatively quick degradation in the environment, it is considered a ‘pseudo-persistent’ compound, which means that environmental concentrations are replenished by continuous disposal,” says Gabrielle Quadra, with the Laboratório de Ecologia Aquática at the Universidade Federal de Juiz de Fora in Brazil.

Though chocolate contains less caffeine than other products such as coffee, all of the small amounts excreted add up with potential environmental consequences, especially when wastewater treatment is inefficient or unavailable. Photo credit: cacaobug on

In a study published last year, Quadra and her team found that high concentrations of caffeine caused skeletal deformations and reduced growth in South America’s endemic catfish (Rhamdia quelen). “These morphological changes can potentially affect larval mobility and swimming capacity, which may affect, consequently, the search for resources and escaping predators,” she states.

“Though the risks were currently low in most of the countries evaluated, increased caffeine consumption, coupled with the lack of sanitation in many countries pose a threat of sublethal morphological effects to local fish species,” concludes Quadra.

A team of researchers led by Davide Seveso, a marine biologist at Italy’s University of Milano-Bicocca, identified caffeine among a variety of other pollutants in reef sponges on Magoodhoo Island in the Maldives. Sponges filter water to feed. In doing so, they accumulate pollutants, says Seveso, making them a good bioindicator. With a human population of around 800, Magoodhoo Island has no wastewater treatment, so waste leaks into the ocean from a coastal landfill. Initial results from another field study done on a resort island, also in the Maldives, showed lower caffeine concentrations in sponges due to wastewater treatment. But analysis reveals that even these reduced concentrations could be having a “potential negative effect” on the sponges, says Seveso.

These impacts on individual species may have wider implications at the ecosystem level, but those broader effects are as yet unstudied. “It is essential to realize that the effects on a specific organism are not limited to that [species alone],” says Quadra. “Each organism occupies a trophic level in the ecosystem, which means that, by affecting [particular] organisms, you can also affect their prey or predators indirectly.”

Sampling of sea sponges in the Maldives revealed accumulation of concentrations of caffeine. Image by Davide Seveso.

Spotlight on a wider problem

Earlier this year, the Stockholm Resilience Centre released a paper stating that humanity’s actions have crossed the threshold of a planetary boundary for the release of novel chemical entities into the environment, threatening Earth’s “safe operating space.” Seen in this context, caffeine is just one among many tens of thousands of human pollutants expelled into the environment daily — contaminants that are mixing and interacting in utterly unknown ways.

In fact, caffeine is so ubiquitous that it is used as a tracer by scientists to identify human waste pollution. Where caffeine is detected, there is often a toxic cocktail of other pollutants such as pharmaceuticals, microplastics and fecal matter. Seveso’s study on sea sponges in the Maldives picked up other pollutants originating from antidepressants and personal care products. These caffeine tracer studies have identified pollution problems across the world, including in lagoons in Mexico and on coral reefs in American Samoa.

The recent sampling of the planet’s rivers found the highest caffeine detection frequencies in Asia (71% of 234 freshwater sampling sites), South America (69.2% of 92 sites), and North America (64.7% of 118 sites). Average concentrations were 4,850, 3,290, and 1,500 nanograms per liter, respectively. Rivers in Africa had a lower detection frequency at 43.8% of sites, with an average concentration of 4,090 nanograms per liter. Rivers polluted with caffeine were widespread, and found near Accra in Ghana, Lahore in Pakistan, Lisbon in Portugal, and Dallas in the United States.

As a stimulant, caffeine can be effective against health problems such as tension headaches and is often contained in over-the-counter cold or flu medications. Inappropriate disposal of medications, down the sink or toilet for example, is considered another possible route for caffeine dispersal in aquatic environments. Photo credit: Michal Jarmoluk via Pixabay.

“Some studies already showed effects to fish behavior, species composition and oxidative stress in aquatic organisms at those concentrations, mainly related to chronic and mixture exposure, but more investigation is needed,” Quadra says.

Despite growing knowledge of caffeine environmental impacts, a vast number of questions remain. How caffeine interacts with other pollutants is unknown, for example, as are the potential influences of two planetary boundaries — climate change and ocean acidification — on the intensification of caffeine impacts. There are, however, indications that some mix of influences — increasing temperature and acidification, for example — may heighten caffeine’s harmful effects. Scientists say not near enough is known to assess the situation properly.

“Our knowledge about caffeine environmental impacts is yet scarce, but some studies demonstrate adverse effects,” says Quadra. “So, why do we need to wait to reach an irreversible situation to control it?”

Dumping out leftover beverages, including energy drinks, soda or coffee, can add to the caffeine burden heading to wastewater facilities. Image by Herbich via Pixabay.

Banner image: A girl drinking coffee. Image by Eric Horst via Flickr (CC BY-NC-SA 2.0).


Wilkinson, J. L., Boxall, A. B. A., Kolpin, D. W., Leung, K. M. Y., Lai, R. W. S,… Teta, C. (2022) Pharmaceutical pollution of the world’s rivers.  Proceedings of the National Academy of Sciences of the United States of America.

Vieira, L. R., Soares, A. M. V. M., Freitas, R. (2022) Caffeine as a contaminant of concern: A review on concentrations and impacts in marine coastal systems. Chemosphere.

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