De-icing salts are chemical compounds applied to paved surfaces to lower the freezing point of water, preventing or melting ice. These products are widely used for public safety on roads and sidewalks during winter weather. However, there is growing public concern regarding their toxicity to living organisms and the environment. The safety of these compounds depends on the specific chemical used, the amount applied, and the target exposed to the residue.
Understanding De-Icing Salt Types
The most common de-icing agent used globally is Sodium Chloride (rock salt). It is highly effective and inexpensive, but only works down to a pavement temperature of about 15°F (-9°C). Other popular alternatives include Calcium Chloride and Magnesium Chloride, which offer superior performance in colder conditions.
Calcium Chloride is notable for its ability to melt ice down to -25°F (-32°C), making it the most reliable choice for extreme cold. Magnesium Chloride works effectively down to approximately -5°F (-21°C) and is considered less damaging to surfaces than calcium chloride. Less common commercial options include Potassium Chloride and Urea, which are sometimes used in specialized products. Their differing chemical properties result in varying levels of corrosive damage and toxicity.
Direct Toxicity Risks to Humans and Companion Animals
The primary health risk associated with de-icing salts involves companion animals, particularly dogs, exposed through walking on treated surfaces. The ionic nature of these salts can draw moisture away from the delicate tissue of paw pads, leading to chemical burns, dryness, and painful cracking. Dogs often exacerbate this risk by licking the salt residue from their paws after a walk, resulting in accidental ingestion.
Ingestion of sodium chloride can lead to serious health issues, including gastrointestinal upset, excessive thirst, and vomiting. Consuming a large quantity can cause hypernatremia, or salt poisoning, which may result in lethargy, tremors, seizures, and potentially death. Toxicosis in a dog can occur with the ingestion of as little as 2 to 3 grams of sodium chloride per kilogram of body weight. For humans, the risk is limited to minor skin or eye irritation from handling the product or from salt-laden splash-back from vehicles.
Environmental Impact on Plants, Soil, and Waterways
The toxicity of de-icing salts significantly impacts roadside vegetation and local freshwater ecosystems. Plants are damaged through two main mechanisms: direct salt spray on foliage and root exposure to high concentrations of salt in the soil. Excess salt in the soil creates a “physiological drought,” making it difficult for roots to absorb sufficient water, leading to desiccation.
The sodium ions in rock salt can displace soil elements, negatively altering the soil structure by reducing aeration and water absorption capacity. This compaction hinders plant growth and long-term soil health. When snowmelt washes salt from paved surfaces, the chloride ions enter local waterways and groundwater, persisting as a pollutant.
Elevated salinity in freshwater streams and ponds is harmful to sensitive aquatic life, impairing the osmoregulation process in organisms like amphibians and fish. Concentrations of chloride as low as 150 milligrams per liter correlate with a drop in aquatic biodiversity. Beyond biological harm, the chloride in de-icers accelerates the corrosion of metal and concrete, contributing to billions of dollars in annual damage to infrastructure, including bridges and roadways.
Minimizing Harm and Safer Alternatives
Users can mitigate the harm caused by traditional chloride-based de-icers by using them judiciously and cleaning up residue after application. Pet owners should immediately wipe their animal’s paws with warm water and a towel after being outside to remove salt residue before ingestion. Applying only the minimum effective amount of de-icer and clearing excess material after the ice has melted reduces environmental runoff.
Safer alternatives exist that function similarly but have fewer toxic side effects. Calcium Magnesium Acetate (CMA) is a popular choice because it is non-corrosive, biodegradable, and safer for both plants and pets, although it is more expensive than rock salt. Other options include organic brines, such as those derived from beet juice or molasses, which can be mixed with chloride salts to reduce the total amount of chloride needed. Simple materials like sand or kitty litter do not melt ice but provide necessary traction and are inert to the environment and animals.