The risk of becoming sick from drinking contaminated water depends on many factors:
- Contaminant - Some contaminants are more harmful than others. Health risks depend on the level and potency of the contaminant and each contaminant may be harmful at different levels.
- Exposure pathway - Health risks may be more or less likely to occur depending on how the contaminant enters the body. For example, people are exposed to drinking water contaminants through ingestion (while drinking or eating food washed with contaminated water), through skin contact (while bathing or swimming in a pool), or through inhalation (while breathing in aerosolized contaminants from humidifiers or in the shower).
- Individual susceptibility - The elderly, children, and pregnant women are often more sensitive to exposure to contaminants or have additional health concerns, and may be more likely to suffer ill effects than the rest of the population.
Access to regulated and treated drinking water also affects exposure to contaminants. According to the US Geological Survey (USGS) Estimated Use of Water in the US in 2010, 93% of California's population, or 34.8 million people, are served by public water systems. The remaining 7% of the state's population rely on water from private wells or other sources. People in unincorporated or rural communities without household connections to public water systems may face more drinking water quality issues than people with access to public systems. Water from unregulated private wells and other sources may not be monitored for contaminants and may pose a greater risk of adverse health effects.
Natural disasters such as earthquakes, floods, and wildfires and human-caused disasters such as chemical spills or construction accidents can damage water treatment facilities and compromise drinking water quality. People living in an area recently impacted by a natural or human-caused disaster may be more vulnerable to health risks from exposure to drinking water contaminants.
The CDC's Safe Water Program addresses water-related issues among these particularly vulnerable populations.