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Propylene Glycol vs. Stoddard Solvent
Propylene Glycol and People 
Propylene glycol is a colorless and odorless liquid and has been established to be biologically safe. The Food and Drug Administration (FDA) has classified it as an additive that is “generally recognized as safe” (GRAS).
and the list goes on. Read the ingredients in your household items and you will be surprised how often everyday we put PG in or on our bodies. It is even used as a lubricant in biomedical implants.
Stoddard Solvent and People 
Stoddard solvent has NOT been established to be biologically safe. The Environmental Protection Agency (EPA) has classified it as a hazardous chemical.
Stoddard solvent smells and tastes like kerosene. Exposure to Stoddard solvent usually occurs when you use a product such as paint thinner that contains it and the vapors get in your lungs and/or eyes. People may be exposed near hazardous waste sites, but it is not known how many are being exposed. It is unclear what routes of exposure are most significant at hazardous waste sites. It is likely exposure will occur via breathing in the air. Although some compounds in Stoddard solvent evaporate quickly, you may be continually exposed near hazardous waste sites if the material is leaking from buried or above ground drums or slowly moving through the soil and seeping through the walls of the basement of a building. Buried, leaky drums may also lead to exposure by touching of contaminated soil or drinking of the ground water. When you breathe in Stoddard solvent it enters the blood stream and the chemical components that make up Stoddard solvent are absorbed by different tissues in the body including the brain. It is not known how quickly the components of Stoddard solvent leave the body. Most information on how Stoddard solvent affects human health comes from studies where exposure is through breathing. The observed physiological effects include:
Point number 7 has very few studies to validate the claim and as a result it is not classified as a carcinogen. However, this is still controversial and despite if it does or does not cause cancer, it is clearly a toxic substance and its use can no longer be justified if there is a suitable replacement for it.
Propylene Glycol and the Environment 
Waste streams from the manufacture of propylene glycol are primarily responsible for the releases into the air, water, and soil. Propylene glycol can enter the environment when it is used as a runway and aircraft de-icing agent. Propylene glycol can also enter the environment through the disposal of products that contains it. It is not likely to exist in large amounts in the air. We have little information about what happens to propylene glycol in the air. The small amounts that may enter the air are likely to break down quickly. If it escapes into the air, it will take between 24 and 50 hours for half the amount released to break down. Propylene glycol can mix completely with water and can soak into soil. It can break down relatively quickly (within several days to a week) in surface water and in soil. Propylene glycol can also travel from certain types of food packages into the food in the package.
Stoddard Solvent and the Environment 
Underground storage of waste is primarily responsible for release of Stoddard solvent into the air, water and soil. Because Stoddard solvent is a mixture of many chemicals, some will evaporate into the air when Stoddard solvent spills onto soils or surface waters. These chemicals may be broken down by sunlight or by other chemicals in the air. Also, some of these chemicals may adsorb to organic matter. Stoddard solvent itself does not dissolve well in water, but some of the chemicals in it do dissolve when it spills on surface water or when it leaks from underground storage tanks. Some of the chemicals in Stoddard solvent can attach to particles in soil and water and in water may sink down to the sediment. In water, soil or sediment microorganisms may break down the chemicals (biodegradation). Although some of the chemicals in Stoddard solvent can attach to organic matter in the soil, if a large amount contaminates the soil, it will move through the soil into groundwater. It is not know whether Stoddard solvent will accumulate in plants or animals living in contaminated soil or water or in animals eating contaminated plants or sediments. However, some of the chemicals that make up Stoddard solvent mixture might accumulate in these situations depending upon the type of chemical. Generally smaller alkanes do not tend to bioaccumulate while aromatics and larger alkanes including some cycloalkanes tend to bioaccumulate.