Silica is a naturally occurring compound composed of silicon and oxygen, known as silicon dioxide (\(\text{SiO}_2\)). Silicon is the second most abundant element in the Earth’s crust, which is why this compound is present in virtually all water sources. Seeing “silica” on a water quality report often raises questions about its safety for consumption. This concern usually stems from a lack of understanding about the specific form this substance takes when dissolved in water. This article evaluates the health impact of consuming the naturally occurring forms of silica found in drinking water.
Forms and Natural Occurrence in Water
Silica enters the water supply through a geological process called weathering, where water passes over and through rocks and soil. The earth’s crust is rich in silicate minerals, such as quartz, granite, and sandstone. As water interacts with these formations, it dissolves trace amounts of the compound. This natural dissolution is the primary mechanism by which silica becomes a constituent of groundwater and surface water.
Once dissolved, silica primarily exists in a soluble form known as monomeric silicic acid (\(\text{Si}(\text{OH})_4\)), also referred to as reactive silica. This is the bioavailable form that the body can absorb. Water can also contain colloidal silica, which consists of ultra-fine, non-dissolved particles suspended in the water.
The concentration of silica in natural waters varies widely depending on the local geology. In most surface water sources, the concentration commonly falls within a range of 5 to 25 milligrams per liter (\(\text{mg/L}\)). In certain groundwater sources, particularly deep wells, concentrations can sometimes exceed 100 \(\text{mg/L}\).
Physiological Safety and Health Implications
Ingested dissolved silica demonstrates a high safety profile for human consumption. The dissolved form is chemically inert and is not readily absorbed by the body. Regulatory bodies like the U.S. Environmental Protection Agency (EPA) and Health Canada have not established a maximum contaminant level for silica in drinking water, indicating it is not regarded as a health hazard at typical environmental concentrations.
Any silica absorbed into the bloodstream is characterized by rapid renal excretion. The body efficiently filters the compound through the kidneys, resulting in low bioavailability and preventing accumulation in tissues. For example, the Australian Drinking Water Guidelines suggest silicon concentrations should not exceed 100 \(\text{mg}\) of silicon per liter (equivalent to 210 \(\text{mg}\) of \(\text{SiO}_2\) per liter), a level far above what is commonly found in drinking water.
Beyond its general safety, research suggests that consuming dissolved silica may offer beneficial physiological roles. One area of focus is its contribution to bone health, as silicon is a component of connective tissues and supports bone mineral density. Silicon also plays a part in the synthesis of collagen, a structural protein important for the integrity of skin, hair, and nails.
Dissolved silica may also chelate aluminum within the digestive tract. Soluble silicic acid binds to aluminum, forming a complex that is less easily absorbed by the gut. This process encourages the excretion of aluminum through the urine, reducing the body’s aluminum burden.
Distinguishing Ingestion Safety from Other Exposures
Public concern about silica often arises from confusion between the dissolved form in water and the hazards associated with industrial exposures. The toxicity concerns relate specifically to the inhalation of fine, crystalline silica dust, such as quartz, which is a known occupational hazard. This respirable crystalline silica is composed of tiny particles that can penetrate deep into the lungs.
When inhaled over long periods, these crystalline particles can cause severe, irreversible lung disease, including silicosis and lung cancer. The rigid and insoluble physical structure of crystalline silica makes it highly reactive and damaging to lung tissues. This is a completely different scenario from the amorphous, dissolved silica found in water.
The silica consumed in drinking water is in a soluble or amorphous (non-crystalline) state, lacking the rigid, damaging structure of crystalline dust. The digestive system is well-equipped to handle and pass the ingested dissolved compound without adverse effects. Dissolved silica is not associated with the lung pathologies linked to occupational crystalline silica exposure.