The natural landscape is a dynamic place, and therefore, the water sources that flow through it are ever-changing in their quality. While a wilderness source may seem perfectly clear and clean, it can be carrying microscopic pathogens---harmful, disease-causing organisms that are invisible to the naked eye. Water sources near popular backcountry camp spots are higher in risk due to the high human traffic. Other sources, such as glacier streams in the alpine, where there's little human or animal presence can actually be pretty low-risk. Still, the simple truth remains: The only real way to verify that your water is safe to drink is to treat it. And the effort it takes to treat water is minor compared to the complications of illness. What are the risks in undeveloped sources? Undeveloped water sources can contain both visible plant and earth matter, such as tannins and sediment, and invisible threats such as metals, fertilizers, and organisms. Of these, the organisms—the microscopic bugs—pose the immediate threat to human health. (Tannins and sediment aren’t particularly harmful; and metals, fertilizers and mercury take repeated long-term exposure to cause harm.) Therefore, those microscopic pests are the primary focus of water treatment technologies. There are four main types of these pathogenic bugs that are of concern swimming in our undeveloped water supplies. Each enters the water primarily through human or animal waste, but they range in size (measured in microns) and vulnerabilities to treatment devices:

• Protozoa Common types: Giardia, Cryptosporidium and Amoebae These single-celled organisms are found in all types of water and are transferred through human and animal feces. Due to their “large” size (typically 5-100 microns), they are easily filtered out. They’re also susceptible to UV and boiling, but can be resistant to chemicals.
• Bacteria Common types: E.coli, Salmonella, Cholera Bacteria are single-celled organisms that are medium in size (typically 0.1-1 micron). Bacteria exist naturally in the water, but harmful species are introduced through feces. These bugs can be filtered out with devices that have met the requirements for bacterial removal, or they can be deactivated through chemicals, UV light or boiling.
• Viruses Common types: Hepatitis A, Norwalk and Rotavirus Viruses are the smallest of the waterborne pathogens. They are species-specific and therefore, the ones that infect humans are transmitted through human feces. They are a primary concern in developing nations with poor sanitation and popular camping zones. Because of their tiny size (typically 0.01-0.1 micron), they require treatment devices that meet the standards for viral removal or deactivation. Typically, the ability of device to also remove/deactivate viruses is what distinguishes it as a water “purifier” versus a water “filter.”
• Parasitic Worms Common types: Hookworm, Roundworm These parasites are a concern primarily in the developing regions. Because they are larger in size (25+ microns), they are easily removed with even the most basic filter and they can be deactivated by boiling. But they are resistant to UV and chemicals.

How do I know if these threats are present where I’m going? There’s no way to know which of these threats---if any---will exist in a particular water source. However, there are some generalizations that offer a starting point for determining an appropriate treatment device. Then, you can dive deeper into your particular priorities and what the conditions will demand:

• Backcountry Travel: The water gathered from wilderness sources—streams, lakes, ponds or rivers—flows through relatively pristine landscapes. Therefore, the organism concentration brought by humans and animal traffic is typically low. The primary threats here are bacteria and protozoa, so a water filter is often adequate.
  • However, you must be observant. The high alpine with no traffic may be nearly risk-free. But an alpine zone with lots of toilet paper—indicating sloppy etiquette and sanitation—may pose risk of human-spred viruses in addition to bacteria and protozoa. Therefore, a purifier might be the safer tool in such an instance. Especially in the backcountry, where the risk gradient can change, it's important to be observant of your surroundings, including what's upstream from you.
• Developing World: Water collected from urban or municipal sources that do not offer adequate treatment can be highly concentrated with pathogens from humans, animals, agriculture and industry. Bacteria, protozoa and viruses are of serious concern, and parasites may be present. Unlike the backcountry where water can be visibly “dirty,” here, contaminated water may be perfectly clear, but not clean. An example is water from a tap in Dhaka, Bangladesh. It's for these reasons that a purifier is often recommended when traveling internationally.
• Disaster Situations: When the home water supply is disrupted and water must be collected from undeveloped and developed sources that have been compromised, pathogens can be highly concentrated. Bacteria, protozoa, viruses and parasites are all of concern. During disasters, the ability to both filter out sediment and to purify that water of viruses is crucial. Therefore, a mechanical purifier or a combination of treatment devices is best to stock in your emergency kit.

Overall, should I choose a water purifier or water filter? To answer that question you must consider where you’re going and your personal tolerance for risk. The safest answer is: If you’re unsure of the type of water you’ll find and the environment it’s flowing through, a purifier or a combination of filter and purifying agent will offer the highest level of protection. That’s why these are suggested for international travel to developing countries, and emergency kits. But they’re also recommended for any place where human waste may not be disposed of properly, or when a member of your party is particularly vulnerable to sickness. If those criteria don’t apply, a water filter is typically adequate for zones that are considered low-risk for viruses, such as the wildernesses of North America. Stay tuned to The Summit Register as we dive deeper into water treatment options, the science behind the bugs and the best tools for the various scenarios you may encounter . To see just how we test the safety of MSR’s water treatment products, take a look inside MSR's world-class microbiology research lab.