The health impacts of harmful algal blooms are extremely varied, ranging from minor skin irritation from bathing in contaminated waters, through to paralysis and even death when seafood contaminated with toxins from harmful algal blooms are consumed.

There are difficulties in determining the true incidence of harmful algal bloom-related illnesses. This is because there is often a lack of experience in public health and medical communities in recognizing, diagnosing and treating patients exposed to harmful algal blooms. In addition, there is a likely huge under-reporting of cases.

The health effects of exposure to harmful algal blooms include:

• Diarrhea, nausea and vomiting
• Neurological symptoms such as loss of sensation, confusion and paralysis
• Muscle aches, joint pain and tremors
• Hepatitis
• Respiratory irritation
• Eye, nose and skin irritation
• Weakness and fatigue
• Acute gastrointestinal problems such as abdominal pain
• Cardiovascular problems such as hypotension (low blood pressure) and bradycardia (slow heart rate)
• Mental health effects such as anxiety, depression, hysteria, neurosis and mental inefficiencies)

In addition to direct risk to human health, harmful algal blooms also impact other aspects of human well-being, such as tourism, medical costs, and local economies through negative impacts on aquaculture industries.

Direct impacts to human health can occur from exposure to toxins through eating contaminated seafood, inhaling contaminated aerosols during water-related recreational or occupational activities, and drinking contaminated water.

Algal blooms can indirectly impact human health by depleting oxygen levels in oceans and lakes or prevent sunlight from reaching deeper water, which can kill fish and negatively impact fisheries, local economies and coastal communities and disrupt livelihoods.

Harmful algal blooms are a global phenomenon – nearly every coastal region of the world is affected by harmful algal blooms, which also occur in the open ocean, brackish and freshwater ecosystems.

Those living near or visiting coastal areas where harmful algal blooms are frequent are at risk of the negative health impacts of algal blooms. People who are particularly susceptible include:

• The elderly
• Those who are overweight, who may have a greater capacity for toxin storage and their symptoms may last longer and be of increased severity ^{3 3}Ciguatera on Nauai: investigation of factors associated with severity of illness. Katz, Terrell-Perica, Sasaki. 1993X
• Children, who due to their lower body weight whose development may be disproportionately affected by toxin exposure. Children are also more likely to participate in risky behaviours and increase their exposure to harmful algal blooms and swallow more water than adults whilst swimming
• Those with asthma are at risk of more severe and longer-lasting symptoms, such as respiratory distress. ^{4 4}Florida red tide toxins (brevetoxins) and longitudinal respiratory effects in asthmatics. Bean et al. 2011X

The incidence of harmful algal blooms is expected to increase with climate change, as growth rates of cyanobacteria and algae are optimal at warmer temperatures and increased CO2 levels.^{5 5}Harmful algal blooms: Future threats in a warmer world. In: Environmental Pollution and its Relation to Climate Change. Backer et al. 2010X ^{6 6}Rising CO2 Levels Will Intensify Phytoplankton Blooms in Eutrophic and Hypertrophic Lakes. Verspagen, J.M.H., et al. 2014X     Toxin production by harmful algal species may also be higher under optimal climate conditions, such as warmer temperatures and nutrient concentrations. ^{7 7}Temperature-related changes in polar cyanobacterial mat diversity and toxin production. Nature Climate Change. Kleinteich, J., et al. 2012X

In addition, changes in the seasonal windows will allow blooms to begin earlier in the year and persist for longer periods of time in the Northern Hemisphere.^{8 8}TITLEX    Evidence has already demonstrated that the number and intensity of harmful algal blooms, and expansions in their geographic range, has increased worldwide.^{9 9}A review of harmful algal blooms and their apparent global increase. Hallegraeff, G.M. 1993X

Increasingly frequent severe weather events, such as hurricanes, changes to ocean circulation patterns and increased ocean acidification as a result of increased atmospheric CO2 levels may also favour the proliferation of harmful algal species.^{10 10}Progress in understanding harmful algal blooms: Paradigm shifts and new technologies for research, monitoring and management. Anderson et al 2012X

Increasing eutrophication through ongoing development and increased resource pressure is also likely to exacerbate the global harmful algal bloom problem,^{11 11}Eutrophication and Harmful Algal Blooms: A Scientific Consensus. Heisler, J. P. et al. 2008X   with the potential for increased proliferation of freshwater and marine cyanobacteria a major concern.^{12 12}The rise of harmful cyanobacteria blooms: the potential roles of eutrophication and climate Change. O’Neill JM, Davis TW, Burford MA, Gobler CJ. 2012X    However, the response of algal blooms to climate change will likely be highly species-specific, given the vast diversity of organisms that cause harmful algal blooms.

Managing the health impacts of harmful algal blooms is complicated, as it requires dealing with multiple toxins, multiple species, multiple toxic fisheries resources, and large and small-scale events that occur intermittently.

Forecasting and early warning
Forecasting can provide officials with advance warning that allows for example, the testing of potentially affected shellfish beds more precisely and for shorter periods of time. They can then post advisories in coastal areas where there is a health risk. Forecasts often rely on satellite imagery, combined with field observations, model outputs and dissemination of information through public health reports.

Monitoring and toxin identification
Continued monitoring of coastal and freshwater ecosystems where harmful algal blooms are frequent can aid in the timely detection of a bloom and alert the public to the potential health impacts. Algal blooms can be monitored through environmental observations of, for example water temperatures, turbidity, salinity, as well as oxygen and nutrient concentrations. Other monitoring techniques include measuring surface colour and reflectance of water samples.

Community support
Programs such as the NOAA HAB Event Response Program provide immediate assistance for managing HAB events by offering technology and expertise, providing supplemental financial support for investigating events, and ensuring events are properly documented. Through this program, NOAA helps to minimize human health risks, assists in identifying the causes of marine animal mortalities, offers training opportunities for managers, and sets baseline conditions for new or reemerging harmful blooms.

Fertiliser management through control of nutrient sources that can stimulate blooms, such as sewage and agricultural and aquacultural runoff can help prevent proliferation of harmful algal blooms.