The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (September 2017) (Learn how and when to remove this template message)
A harmful algal bloom (HAB) are organisms that can severely lower oxygen levels in natural waters, killing marine life. Some HABs are associated with algae-produced toxins. Blooms can last from a few days to many months. After the bloom dies, the microbes which decompose the dead algae use up even more of the oxygen, which can create fish die-offs. When these zones of depleted oxygen cover a large area for an extended period of time, they are referred to as dead zones, where neither fish nor plants are able to survive.
HABs are induced by an overabundance of nutrients in the water. The two most common nutrients are fixed nitrogen (nitrates, ammonia, urea) and phosphate. These nutrients are emitted by agriculture and other industries. Higher water temperature and low circulation also contribute. HABs can cause significant harm to animals, the environment and economies. They have been increasing in size and frequency worldwide, a fact that many experts attribute to global climate change. The U.S. National Oceanic and Atmospheric Administration (NOAA) predicts more harmful blooms in the Pacific Ocean.
HABs from blue-green algae (cyanobacteria) can appear as a foam, scum, or mat on or just below the surface of water and can take on various colors depending on their pigments. Blue-green algae blooms in freshwater lakes or rivers may appear bright green, often with surface streaks which looks like floating paint. Similarly, red tides made up of dinoflagellates, also contain photosynthetic pigments that vary in color from green to brown to red.
Most blooms occur in warm waters that have excessive nutrients. The harmful effects from such blooms is due to the toxins they produce or from using up oxygen in the water which can lead to fish die-offs.
Not all algal blooms are harmful, however, with some only discoloring water, producing a smelly odor, or adding a bad taste to the water. Unfortunately, it is not possible to tell if a bloom is harmful from just appearances, since sampling and microscopic examination is required.
There are three main types of algae which can form into harmful algal blooms: cyanobacteria, dinoflagellates and diatoms. All three are made up of microscopic floating organisms which, like plants, can create their own food from sunlight by means of photosynthesis. That ability makes them an essential part of the food web for small fish and other organisms.:246
Harmful algal blooms in freshwater lakes and rivers, or at estuaries, where rivers flow into the ocean, are caused by blue-green algae, also known as cyanobacteria. They can produce hazardous toxins, such as microcystins, a neurotoxin which destroys nerve tissue of mammals. In high enough concentrations, water treatment plants may be unable to remove the toxin and will advise residents to avoid drinking tap water, as happened in Toledo, Ohio in August 2014.
They also cause harm by blocking the sunlight or by using up the oxygen needed by fish or plant life, which can lead to fish die-offs. When such oxygen-depleted water covers a large area for an extended period of time, it can become hypoxic, commonly called a dead zone. These dead zones can be the result of numerous different factors ranging from natural phenomenon to deliberate human interaction. These dead zones are not just limited to large bodies of fresh water as found in the great lakes, but are also prone to bodies of salt water as well. 
The other types of algae are diatoms and dinoflagellates, found primarily in marine environments, such as ocean coastlines or bays, where they can also form algal blooms, commonly called red tides. Red tides, however, may be a natural phenomenon, although when they form close to coastlines or in estuaries. They can occur when warmer water, salinity, and nutrients reach certain levels, which then stimulates their growth. Most red tide algae are dinoflagellates. They are visible in water at a concentration of 1,000 algae cells per milliliter, while in dense blooms they can measure over 200,000 per milliliter.
Diatoms produce domoic acid, another neurotoxin, which can cause seizures in higher vertebrates and birds as it concentrates up the food chain. Domoic acid readily accumulates in the bodies of shellfish, sardines, and anchovies, which if then eaten by sea lions, otters, cetaceans, birds or people, can affect the nervous system causing serious injury or death. In the summer of 2015, the state governments closed important shellfish fisheries in Washington, Oregon and California because of high concentrations of domoic acid in shellfish.
Among the causes of algal blooms are:
Climate change, according to NOAA scientists, contributes to warmer waters which makes conditions more favorable for algae growth in more regions and farther north. Global warming is also considered a key factor for algal blooms in the Southern hemisphere, acknowledged by scientists in Australia. In general, still, warm, shallow water, combined with high-nutrient conditions in lakes or rivers, increases the risk of harmful algal blooms.
Nutrients enter freshwater or marine environments as surface runoff from agricultural pollution and urban runoff from fertilized lawns, golf courses and other landscaped properties; and from sewage treatment plants that lack nutrient control systems. Additional nutirents are introduced from atmospheric pollution. Coastal areas worldwide, especially wetlands and estuaries, coral reefs and swamps, are prone to being overloaded with those nutrients. Most of the large cities along the Mediterranean Sea, for example, discharge all of their sewage into the sea untreated. The same is true for most coastal developing countries.
In the U.S., such runoff, despite being the largest source of nutrients added to rivers and lakes, is mostly unregulated under the federal Clean Water Act. Locally developed initiatives to reduce nutrient pollution are underway in various areas of the country, such as the Great Lakes region and the Chesapeake Bay. To help reduce algal blooms in Lake Erie, the State of Ohio presented a plan in 2016 to reduce phosphorus runoff.
The Chesapeake Bay, the largest estuary in the U.S., has suffered from repeated large algal blooms for decades due to chemical runoff from multiple sources, including 9 large rivers and 141 smaller streams and creeks in parts of six states. In addition, the water is quite shallow and only 1% of the waste entering it gets flushed into the ocean.
By weight, 60% of the phosphates entering the bay in 2003 were from sewage treatment plants, while 60% of its nitrates came from fertilizer runoff, farm animal waste, and the atmosphere. About 300 million pounds (140 Gg) of nitrates are added to the bay each year. The population increase in the bay watershed, from 3.7 million people in 1940 to 18 million in 2015 is also a major factor, as economic growth leads to the increased use of fertilizers and rising emissions of industrial waste.
The six states and the local governments in the Chesapeake watershed have upgraded their sewage treatment plants to control nutrient discharges. The U.S. Environmental Protection Agency (EPA) estimates that sewage treatment plant improvements in the Chesapeake region between 1985 and 2015 have prevented the discharge of 900 million pounds (410 Gg) of nutrients, with nitrogen discharges reduced by 57% and phosphorus by 75%. Agricultural and urban runoff pollution continue to be major sources of nutrients in the bay, and efforts to manage those problems are continuing throughout the 64,000 square miles (170,000 km2) watershed.
As algal blooms grow, they deplete the oxygen in the water and block sunlight from reaching fish and plants. Such blooms can last from a few days to many months. With less light, plants beneath the bloom can die and fish can starve.Furthermore, the dense population of a bloom reduces oxygen saturation during the night by respiration. And when the algae eventually die off, the microbes which decompose the dead algae use up even more oxygen, which in turn causes more fish to die or leave the area. When oxygen continues to be depleted by blooms it can lead to hypoxic dead zones, where neither fish nor plants are able to survive. These dead zones in the case of the Chesapeake Bay, where they are a normal occurrence, are also suspected of being a major source of methane.
The negative impact on fish can be even more severe when they are confined to pens, as they are in fish farms. In 2007 a fish farm in British Columbia lost 260 tons of salmon as a result of blooms, and in 2016 a farm in Chile lost 23 million salmon after an algal bloom.
According to NOAA, less than one percent of algal blooms produce hazardous toxins, such as microcystins. Although blue-green or other algae do not usually pose a direct threat to health, the toxins (poisons) which they produce are considered dangerous to humans, land animals, sea mammals, birds and fish when the toxins are ingested. The toxins are neurotoxins which destroy nerve tissue which can affect the nervous system, brain, and liver, and can lead to death. Tests have shown some toxins near blooms can be in the air and thereby be inhaled, which could affect health.
There is no treatment available for animals, including livestock cattle, if they drink from algal blooms where such toxins are present. The Florida Department of Health recommends that people and pets be kept away from algal blooms to avoid contact.
Eating fish or shellfish from lakes with a bloom nearby is not recommended. A study has shown that algal toxins may be the cause for as many as 60,000 intoxication cases in the world each year. This is due to the accumulation of potent toxins in shellfish that consume those algae and then these shellfish are later consumed by humans which may result in Amnesic shellfish poisoning, Diarrhetic shellfish poisoning, Neurotoxic shellfish poisoning, and Paralytic shellfish poisoning. Toxic paralytic shellfish poisoning in the Philippines during red tides have caused at least 120 deaths over a few decades. After a HAB in Monterey Bay, California, health officials warned people not to eat certain parts of anchovy, sardines, or crab caught in the bay.In 1987 a new illness had emerged which was called amnesic shellfish poisoning. People who had eaten mussels from Prince Edward Island were found to have amnesic shellfish poisoning. The illness was caused by domoic acid, produced by a diatom found in the area where the mussels were cultivated. In 2015 most shellfish fisheries in Washington, Oregon and California were shut down because of high concentrations of toxic domoic acid in shellfish. People have been warned that inhaling vapors from waves or wind during a red tide may cause asthma attacks or lead to other respiratory ailments.
Agricultural officials in Utah worried that even crops could become contaminated if irrigated with toxic water, although they admit that they can't measure contamination accurately because of so many variables in farming. They issued warnings to residents, however, out of caution.
Persons are generally warned not to enter or drink water from algal blooms, or let their pets swim in the water since many pets have died from algal blooms. In at least one case, people began getting sick before warnings were issued.
In some locations visitors have been warned not to even touch the water. Boaters have been told that toxins in the water can be inhaled from the spray from wind or waves. Ocean beaches, lakes and rivers have been closed due to algal blooms. After a dog died in 2015 from swimming in a bloom in California's Russian River, officials likewise posted warnings for parts of the river. Boiling the water at home before drinking does not remove the toxins.
Scientists in Britain, which has seen a huge increase in toxic algae, suspect that drinking water from sources that have blue-green algae may contribute to Alzheimer's, Parkinson's or Lou Gehrig's Disease. Few water treatment plants regularly test for cyanobacterial toxins, however.
In August 2014 the city of Toledo, Ohio advised its 500,000 residents to not drink tap water as the high toxin level from an algal bloom in western Lake Erie had affected their water treatment plant's ability to treat the water to a safe level. The emergency required using bottled water for all normal uses except showering, which seriously affected public services and commercial businesses. The bloom returned in 2015 and was forecast again for the summer of 2016.
In 2004, a bloom in Kisumu Bay, which is the drinking water source for 500,000 people in Kisumu, Kenya, suffered from similar water contamination. In China, water was cut off to residents in 2007 due to an algal bloom in its third largest lake, which forced 2 million people to use bottled water. A smaller water shut-down in China affected 15,000 residents two years later at a different location. Australia in 2016 also had to cut off water to farmers.
Alan Steinman of Grand Valley State University has explained that among the major causes for the algal blooms in general, and Lake Erie specifically, is because blue-green algae thrive with high nutrients, along with warm and calm water. Lake Erie is more prone to blooms because it has a high nutrient level and is shallow, which causes it to warm up more quickly during the summer.
Symptoms from drinking toxic water can show up within a few hours after exposure. They can include nausea, vomiting, and diarrhea, or trigger headaches and gastrointestinal problems. Although rare, liver toxicity can cause death. Those symptoms can then lead to dehydration, another major concern. In high concentrations, the toxins in the algal waters when simply touched can cause skin rashes, irritate the eyes, nose, mouth or throat. Those with suspected symptoms are told to call a doctor if symptoms persist or they can't hold down fluids after 24 hours.
The hazards which accompany harmful algal blooms have hindered visitors' enjoyment of beaches and lakes in places in the U.S. such as Florida, California Vermont, and Utah. Persons hoping to enjoy their vacations or days off have been kept away to the detriment of local economies. Lakes and rivers in North Dakota, Minnesota, Utah, California and Ohio have had signs posted warning about the potential of health risk.
In July 2016 Florida declared a state of emergency for four counties as a result of blooms. They were said to be "destroying" a number of businesses and affecting local economies, with many needing to shut down entirely. Some beaches were closed, and hotels and restaurants suffered a drop in business. Tourist sporting activities such as fishing and boating were also affected. Senator Marco Rubio called the situation in Florida "a health, ecological and economic emergency."
Similar blooms have become more common in Europe, with France among the countries reporting them. In the summer of 2009, beaches in northern Brittany became covered by tonnes of potentially lethal rotting green algae. A horse being ridden along the beach collapsed and died from fumes given off by the rotting algae.
The economic damage resulting from lost business has become a serious concern. According to one report in 2016, the four main economic impacts from harmful algal blooms come from damage to human health, fisheries, tourism and recreation, and the cost of monitoring and management of area where blooms appear. EPA estimates that algal blooms impact 65 percent of the country's major estuaries, with an annual cost of $2.2 billion. In the U.S. there are an estimated 166 coastal dead zones. Because data collection has been more difficult and limited from sources outside the U.S., most of the estimates as of 2016 have been primarily for the U.S.
In port cities in the Shandong Province of eastern China, residents are no longer surprised when massive algal blooms arrive each year and inundate beaches. Prior to the Beijing Olympics in 2008, over 10,000 people worked to clear 20,000 tons of dead algae from beaches. In 2013 another bloom in China, thought to be its largest ever, covered an area of 7,500 square miles, and was followed by another in 2015 which blanketed an even greater 13,500 square miles. The blooms in China are thought to be caused by pollution from untreated agricultural and industrial discharges into rivers leading to the ocean.
As early as 1976 a short-term, relatively small, dead zone off the coasts of New York and New Jersey cost commercial and recreational fisheries over $500 million. In 1998 a red tide in Hong Kong killed over $10 million in high-value fish.
In 2009, the economic impact for the state of Washington's coastal counties dependent on its fishing industry was estimated to be $22 million. In 2016, the U.S. seafood industry expected future lost revenue could amount to $900 million annually.
NOAA has provided a few cost estimates for various blooms over the past few years: $10.3 million in 2011 due to the red tide at Texas oyster landings; $2.4 million lost income by tribal commerce from 2015 fishery closures in the pacific northwest; $40 million from Washington state's loss of tourism from the same fishery closure.
Along with damage to businesses, the toll from human sickness results in lost wages and damaged health. The costs of medical treatment, investigation by health agencies through water sampling and testing, and the posting of warning signs at effected locations is also costly.
The closures applied to areas where this algae bloom occurs has a big negative impact of the fishing industries, add to that the high fish mortality that follows, the increase in price due to the shortage of fish available and decrease in the demand for seafood due to the fear of contamination by toxins.This causes a big economic loss for the industry
Economic costs are estimated to rise. In June 2015, for instance, the largest known toxic HAB forced the shutdown of the west coast shellfish industry, the first time that has ever happened. One Seattle NOAA expert commented, "This is unprecedented in terms of the extent and magnitude of this harmful algal bloom and the warm water conditions we're seeing offshore...." The bloom covered a range from Santa Barbara, California northward to Alaska.
The number of reported harmful algal blooms (cyanobacterial) has been increasing throughout the world. In the U.S., every coastal state has had harmful algal blooms over the last decade, and species have emerged in new locations that were not previously known to have problems. Inland, major rivers have seen an increase in their size and frequency. In 2015 the Ohio River had a bloom which stretched an "unprecedented" 650 miles (1,050 km) into adjoining states and tested positive for toxins, which created drinking water and recreation problems. A portion of Utah's Jordan River was closed due to toxic algal bloom in 2016.
Researchers have reported the growth of HABs in Europe, Africa and Australia. Those have included blooms on some of the African Great Lakes, such as Lake Victoria, the second largest freshwater lake in the world.India has been reporting an increase in the number of blooms each year. In 1977 Hong Kong reported its first red tide. By 1987 they were getting an average of 35 per year. Additionally, there have been reports of harmful algal blooms throughout popular Canadian lakes such as Beaver Lake and Quamichan Lake. These blooms were responsible for the deaths of a few animals and led to swimming advisories. 
Global warming and pollution is causing algal blooms to form in places previously considered "impossible" or rare for them to exist, such as under the ice sheets in the Arctic, in Antarctica, the Himalayan Mountains, the Rocky Mountains, and in the Sierra Nevada Mountains.
Massive fish die-offs have been caused by HABs. In 2016, 23 million salmon which were being farmed in Chile died from a toxic algae bloom. To get rid of the dead fish, the ones fit for consumption were made into fishmeal and the rest were dumped 60 miles offshore to avoid risks to human health. The economic cost of that die-off is estimated to have been $800 million. Environmental expert Lester Brown has written that the farming of salmon and shrimp in offshore ponds concentrates waste, which contributes to eutrophication and the creation of dead zones.
Other countries have reported similar impacts, with cities such as Rio de Janeiro, Brazil seeing major fish die-offs from blooms becoming a common occurrence. In early 2015, Rio collected an estimated 50 tons of dead fish from the lagoon where water events in the 2016 Olympics were planned to take place.
The Monterey Bay has suffered from harmful algal blooms, most recently in 2015: "Periodic blooms of toxin-producing Pseudo-nitzschia diatoms have been documented for over 25 years in Monterey Bay and elsewhere along the U.S. west coast. During large blooms, the toxin accumulates in shellfish and small fish such as anchovies and sardines that feed on algae, forcing the closure of some fisheries and poisoning marine mammals and birds that feed on contaminated fish." Similar fish die-offs from toxic algae or lack of oxygen have been seen in Russia, Colombia, Vietnam, China, Canada, Turkey, Indonesia, and France.
Land animals, including livestock and pets have been affected. Dogs have died from the toxins after swimming in algal blooms. Warnings have come from government agencies in the state of Ohio, which noted that many dogs and livestock deaths resulted from HAB exposure in the U.S. and other countries. They also noted in a 2003 report that during the previous 30 years, they have seen more frequent and longer-lasting harmful algal blooms." In 50 countries and 27 states that year there were reports of human and animal illnesses linked to algal toxins. In Australia, the department of agriculture warned farmers that the toxins from a HAB had the "potential to kill large numbers of livestock very quickly."
Marine mammals have also been seriously harmed, as over 50 percent of unusual marine mammal deaths are caused by harmful algal blooms. In 1999, over 65 bottlenose dolphins died during a red tide in Florida. In 2013 a red tide in southwest Florida killed a record number of Manatee. Whales have also died in large numbers. During the period from 2005 to 2014, Argentina reported an average 65 baby whales dying which experts have linked to algal blooms. A whale expert there expects the whale population to be reduced significantly. In 2003 off Cape Cod in the North Atlantic, at least 12 humbpack whales died from toxic algae from a red tide. In 2015 Alaska and British Columbia reported many humpback whales had likely died from HAB toxins, with 30 having washed ashore in Alaska. "Our leading theory at this point is that the harmful algal bloom has contributed to the deaths," said a NOAA spokesperson.
Birds have died after eating dead fish contaminated with toxic algae. Rotting and decaying fish are eaten by birds such as pelicans, seagulls, cormorants, and possibly marine or land mammals, which then become poisoned. The nervous systems of dead birds were examined and had failed from the toxin's effect. On the Oregon and Washington coast, a thousand scoters, or sea ducks, were also killed in 2009. ""This is huge," said a University professor. As dying or dead birds washed up on the shore, wildlife agencies went into "an emergency crisis mode."
It has even been suggested that harmful algal blooms are responsible for the deaths of animals found in fossil troves.
According to NOAA, blooms can harm the environment even without producing toxins by depleting oxygen from the water when growing and while decaying after they die. Blooms can also block sunlight to organisms living beneath it. A record-breaking number and size of blooms have formed in the Pacific coast, in Lake Erie, in the Chesapeake Bay and in the Gulf of Mexico, where a number of dead zones were created as a result. In the 1960s the number of dead zones worldwide was 49; the number rose to over 400 by 2008. In the U.S. they are especially prevalent along the east and south coasts.
Various important natural habitats such as rivers, lakes and estuaries have continued to degrade and has contributed to creating more oxygen-deprived dead zones, including some in the Gulf of Mexico, the Chesapeake Bay, and Lake Erie.
Among the largest dead zones were those in northern Europe's Baltic Sea and the Gulf of Mexico, which affects a $2.8 billion U.S. fish industry. Unfortunately, dead zones rarely recover and usually grow in size. One of the few dead zones to ever recover was in the Black Sea, which returned to normal fairly quickly after the collapse of the Soviet Union in the 1990s due to a resulting reduction in fertilizer use.
Although a number of algaecides have been effective in killing algae, they have been used mostly in small bodies of water. For large algal blooms, however, adding algaecides such as silver nitrate or copper sulfate can have worse effects, such as killing fish outright and harming other wildlife. The negative effects can therefore be worse than letting the algae die off naturally.
Other experts have proposed building reservoirs to prevent the movement of algae downstream. However, that can lead to the growth of algae within the reservoir, which become sediment traps with a resultant buildup of nutrients. Some researchers found that intensive blooms in reservoirs were the primary source of toxic algae observed downstream, but the movement of algae has so far been less studied, although it is considered a likely cause of algae transport.
A growing number of scientists agree that there is an urgent need to protect the public by being able to forecast harmful algal blooms. One way they hope to do that is with sophisticated sensors which can help warn about potential blooms. The same types of sensors can also be used by water treatment facilities to help them prepare for higher toxic levels.
The only sensors now in use are located in the Gulf of Mexico. In 2008 similar sensors in the Gulf forewarned of an increased level of toxins which led to a shutdown of shellfish harvesting in Texas along with a recall of mussels, clams and oysters, possibly saving many lives. With an increase in the size and frequency of HABs, experts state the need for significantly more sensors located around the country. The same kinds of sensors can also be used to detect threats to drinking water from intentional contamination.
Four U.S. federal agencies--EPA, the National Aeronautics and Space Administration (NASA), NOAA, and the U.S. Geological Survey (USGS)--are working on ways to detect and measure cyanobacteria blooms using satellite data. The data may help develop early-warning indicators of cyanobacteria blooms by monitoring both local and national coverage. In 2016 automated early-warning monitoring systems were successfully tested, and for the first time proven to identify the rapid growth of algae and the subsequent depletion of oxygen in the water.
However, in the U.S. at least, funding for such warning devices has been shrinking, with approved funding down 45% over the last five years. According to one marine science professor, "We need it more than ever, and we've brought ourselves to the precipice of making great forecasts, but we can't make it happen."
The nitrates and phosphorus in fertilizers cause algal blooms when they run off into lakes and rivers after heavy rains. Modifications in farming methods have been suggested, such as only using fertilizer in a targeted way at the appropriate time exactly where it can do the most good for crops to reduce potential runoff. A method used successfully is drip irrigation, which instead of widely dispersing fertilizers on fields, drip-irrigates plant roots through a network of tubes and emitters, leaving no traces of fertilizer to be washed away. According to the Organisation for Economic Co-operation and Development (OECD), drip irrigation also prevents the formation of algal blooms in reservoirs for drinking water while saving up to 50% of water typically used by agriculture.
A number of states in the U.S. have tried eliminating phosphates in detergent and by cleaning[vague] water treatment plants, which succeeded in reducing the amount that entered Lake Erie by 66%.[not in citation given] However, changes in farming practices during that period increased chemical runoff, thereby offsetting the improvements.
There have also been proposals to create buffer zones of foliage and wetlands to help filter out the phosphorus before it reaches water. Other experts have suggested using conservation tillage, changing crop rotations, and restoring wetlands. "The most important thing that can be done is to reduce agricultural runoff," according to a Great Lakes pollution expert. "Prevention is better than treatment." Another expert states that it is possible for some dead zones to shrink within a year under proper management.
There have been a few success stories in controlling chemicals. After Norway's lobster fishery collapsed in 1986 due to low oxygen levels, for instance, the government in neighboring Denmark took action and reduced phosphorus output by 80 percent which brought oxygen levels closer to normal. Similarly, dead zones in the Black Sea and along the Danube River recovered after phosphorus applications by farmers were reduced by 60%.
In 2008, the U.S. government prepared a report on the problem, "Harmful Algal Bloom Management and Response: Assessment and Plan". The report recognized the seriousness of the problem:
It is widely believed that the frequency and geographic distribution of HABs have been increasing worldwide. All U.S. coastal states have experienced HABs over the last decade, and new species have emerged in some locations that were not previously known to cause problems. HAB frequency is also thought to be increasing in freshwater systems.
The report suggested among other remedies, using improved monitoring methods, trying to improve predictability, and testing new potential methods of controlling HABs. Some countries surrounding the Baltic Sea, which has the world's largest dead zone, have considered using massive geoengineering options, such as forcing air into bottom layers to aerate them.
In 2015, NOAA created 12 new research grants totaling nearly $2.1 million which they would award to national organizations doing research on harmful algal blooms and hypoxia, which they consider to be "two of the most scientifically complex and economically damaging coastal issues."
Most countries, states and large cities have departments which will help monitor and report incidents of algal blooms. The Centers for Disease Control and Prevention (CDC) in the U.S. launched the country's first algal bloom reporting system in June 2016. Environmental agencies in individual U.S. states will accept reports of blooms from citizens and will work with cities to test and report incidents to the media. A few examples: