A look at the real risks of untreated water, using data from the WHO, the CDC, and Costa Rican studies

Treating water correctly—filtering and disinfecting—is the barrier that separates a water source from a public health problem

Water is essential for life, but when not treated correctly, it can become the primary vehicle for disease transmission. The World Health Organization (WHO) estimates that at least 1.7 billion people worldwide consume water contaminated with fecal matter, and that diarrheal diseases associated with inadequate water supply, sanitation, and hygiene cause approximately 505,000 deaths each year.

In this blog, we review which biological and chemical contaminants can be present in untreated water, what health problems they cause, who is most exposed, what the situation is in Costa Rica, and why the combination of filtration and disinfection—not just one or the other—is the correct way to protect the water you consume.

Contaminated water remains one of the leading causes of preventable disease and death in the world

Why can water "look clean" and not be safe?

The clarity of a glass of water says almost nothing about its microbiological or chemical safety. Bacteria, viruses, and protozoan cysts are too small to be seen with the naked eye, and many of the most dangerous chemical contaminants (arsenic, lead, nitrates) have no perceptible taste, odor, or color at concentrations that pose a health risk. Therefore, the only reliable way to know if water is safe is to analyze it in a laboratory and treat it consistently, rather than relying on its appearance.

What you don't see also counts: bacteria, viruses, and protozoa are invisible to the naked eye in water

Biological contaminants and the diseases they cause

Water can harbor different types of microorganisms, each with a different behavior and risk.

Bacteria

Pathogenic bacteria usually reach water through fecal contamination—from poorly treated sewage, latrines near wells, or animal feces—and are responsible for a significant portion of the global diarrheal disease burden:

 

Viruses

Regarding viruses, it is very common to find the following in water:

• Rotavirus: the leading cause of severe diarrhea in children under 5 years old worldwide.
• Norovirus: extremely contagious; can cause outbreaks of acute gastroenteritis even with a very low dose of the virus.
• Hepatitis A: attacks the liver; can cause jaundice, intense fatigue, and, in severe cases, prolonged liver damage.

Protozoa and parasites

At the protozoan and parasite level, the following are very common:

• Giardia lamblia: causes giardiasis—prolonged diarrhea, gas, and nutrient malabsorption—and can persist for weeks if untreated.
• Cryptosporidium: forms cysts (oocysts) that are highly resistant to conventional chlorine doses; causes severe diarrhea, especially dangerous in immunocompromised individuals.
• Entamoeba histolytica: causes amebiasis, which can lead to dysentery and, in severe cases, liver abscesses.

According to the WHO, diarrhea is the third leading cause of death in children aged 1 to 59 months, with 443,832 deaths annually in children under 5. The vast majority of these cases are preventable with properly treated water, adequate sanitation, and hygiene.

Chemical contaminants: the silent risk

Unlike microorganisms, chemical contaminants do not usually cause immediate symptoms. Their damage is cumulative and often manifests years after exposure, making them more difficult to associate with drinking water. These types of contaminants can be present in water even if it looks very clean and transparent.

Arsenic

Naturally present in groundwater in certain geological areas, arsenic is odorless and tasteless. Chronic exposure has been associated with skin lesions and an increased risk of skin, bladder, and lung cancer, in addition to cardiovascular effects.

Lead

It rarely comes from the original water source: it leaches into the water from old pipes, solder, or plumbing fixtures. According to the EPA and the CDC, there is no level of lead exposure considered safe, and its effects are especially serious in children, where it can permanently affect neurological and cognitive development.

Nitrates

They typically come from agricultural runoff (fertilizers), septic tanks, or sewage. In infants under 6 months old, consumption of water with high levels of nitrates can cause methemoglobinemia—known as "blue baby syndrome"—a condition that reduces the blood's ability to transport oxygen.

Disinfection byproducts

Paradoxically, even the disinfection process can generate risks if not managed correctly: when chlorine reacts with organic matter present in water, byproducts such as trihalomethanes (THMs) are formed, which various studies have linked to an increased risk of certain types of cancer in the long term. This is not an argument against disinfection—which remains essential—but in favor of dosing it correctly and considering complementary technologies such as UV light.

Who is most exposed to the risk?

The following groups of people are particularly more vulnerable to the risks described:

• Children under 5 years old: their immune systems are still developing and they are much more vulnerable to dehydration from diarrhea.
• Older adults: more likely to have underlying health conditions that aggravate any infection.
• Immunocompromised individuals: for whom microorganisms like Cryptosporidium pose a considerably greater risk.
• Pregnant women and babies: particularly vulnerable to contaminants like nitrates and lead.
• Communities with limited access to treatment: rural or remote areas outside the public network, which often rely on wells or springs without continuous treatment.

The water situation in Costa Rica

Costa Rica has, in regional terms, relatively high drinking water coverage: according to data published in the Revista Tecnología en Marcha of the Costa Rica Institute of Technology, in 2016, 91.8% of the population had access to drinking water quality, although 99.5% received piped water. That difference implies that, even with near-universal aqueduct coverage, about 400,000 people did not have access to water that met potability standards that same year.

Drinking water quality coverage in Costa Rica and the 2030 national goal

The same study documents a clear correlation between the improvement in drinking water coverage and public health: between 2000 and 2015, while drinking water coverage rose from 75% to over 91%, infant mortality in children under 5 years old dropped from 4.5 to 0.9 per 10,000 inhabitants. Incidents of water contamination with hydrocarbons, nitrates, heavy metals, pesticides, and arsenic were also documented in various parts of the country between 2001 and 2016.

The Ministry of Health and the Costa Rican Institute of Aqueducts and Sewers (AyA) maintain continuous monitoring of parameters such as trihalomethanes in the Greater Metropolitan Area, confirming that levels remain within those permitted by the Regulation for Drinking Water Quality (Decree No. 38924-S). This type of constant vigilance is, precisely, the reason why treating water should not depend solely on the source it comes from.

Why is a single treatment method not enough? Why is a multiple barrier necessary?

It is common to think that it is enough to just "filter" or "add chlorine" to water, but no single method covers all risks:

• Filtration retains particles, sediments, and, depending on the technology, some dissolved contaminants, but it does not guarantee the complete inactivation of the water's biological load on its own.
• Disinfection (chlorine, UV light, ozone) inactivates bacteria, viruses, and protozoa, but does not remove particles, heavy metals, or most dissolved chemical contaminants—and, if poorly dosed, can generate its own byproducts, as explained before with THMs.

That is why modern water treatment systems, both municipal and residential, are designed following the "multiple barrier" principle: several complementary stages that, together, cover the risks that no individual stage covers on its own.

The multiple barrier principle: each stage covers what the previous one does not remove

What can you do to protect your water?

Below are some of the most basic recommendations that should not be ignored:

• Know your water quality: if your source is a well, a spring, or any system other than the monitored public network, perform periodic laboratory analyses. Many of the most dangerous contaminants are not detected by sight, taste, or smell.
• Install filtration appropriate for the real problem: a sediment filter does not solve an arsenic problem, and a system for iron and manganese does not replace disinfection. Treatment must respond to what the water analysis found.
• Ensure a reliable disinfection stage: correctly dosed chlorine, UV light, or any validated disinfection technology, depending on the end use of the water.
• Maintain the equipment: a saturated filter or an expired UV lamp stop protecting, even if the equipment remains installed and "running."
• Do not assume that water is safe just because it looks clean: clarity is not a reliable indicator of microbiological or chemical safety.

Conclusion

Contaminated water remains, today, one of the leading causes of preventable disease in the world, and Costa Rica is not exempt from the challenge: there are still hundreds of thousands of people without access to drinking water quality, and documented episodes of chemical contamination in different areas of the country. The good news is that it is a perfectly manageable risk with the right tools: knowing the quality of the water you consume, filtering it according to the specific problem it has, and disinfecting it reliably. At Hidroteco, we can help you evaluate your water and design the treatment system that corresponds to your situation.

References

García, R. Q. (2025, September 27). Salud y AyA garantizan calidad del agua potable en la GAM [Health and AyA guarantee drinking water quality in the GAM]. Ministerio de Salud Costa Rica. https://www.ministeriodesalud.go.cr/index.php/prensa/62-noticias-2025/2252-salud-y-aya-garantizan-calidad-del-agua-potable-en-la-gam

Mora-Alvarado, D., & Portuguez-Barquero, C. F. (2018). Agua para consumo humano y saneamiento en Costa Rica al 2016. Metas al 2022 y al 2030 [Water for human consumption and sanitation in Costa Rica as of 2016. Goals for 2022 and 2030]. Revista Tecnología En Marcha, 31(2), 72. https://doi.org/10.18845/tm.v31i2.3625

Nitrate in Drinking Water - MN Dept. of Health. (n.d.). https://www.health.state.mn.us/communities/environment/water/contaminants/nitrate.html

UN News. (2014, November 19). Every dollar invested in water, sanitation brings four-fold return in costs – UN. UN News. https://news.un.org/en/story/2014/11/484032

What are some of the health effects of lead in children? | US EPA. (2026, January 21). US EPA. https://www.epa.gov/lead/what-are-some-health-effects-lead-children

World Health Organization: WHO. (2026, June 10). Lead poisoning. https://www.who.int/news-room/fact-sheets/detail/lead-poisoning-and-health

World Health Organization: WHO. (2024, March 7). Diarrhoeal disease. https://www.who.int/news-room/fact-sheets/detail/diarrhoeal-disease

World Health Organization: WHO. (2023, September 13). Drinking-water. https://www.who.int/news-room/fact-sheets/detail/drinking-water