A soap substitute is a natural or synthetic cleaning product used in place of soap or other detergents, typically to reduce environmental impact or health harms or provide other benefits.

Traditionally, soap has been made from animal or plant derived fats and has been used by humans for cleaning purposes for several thousand years. Soap is not harmful to human health but, like any natural or unnatural surfactant, it does have the potential to cause environmental harm by forming a surface film that impedes the diffusion of oxygen into the water if it is added to an aquatic environment faster than it can biodegrade.

Many washing agents today, from laundry and dish detergents to body wash and shampoos, are technically not soap, but synthetic detergents. They also often contain compounds that have been found to be harmful to human and wildlife health as well as to the environment. In this context, “Soap Substitutes” refers to cleansing products that significantly reduce or eliminate some or all of the components that have the potential to cause human or environmental harm. Throughout the last 100 years many changes have been made to the formulas of cleansing agents for these purposes, but the process of developing effective substitute detergent formulations that are completely harmless to humans and the environment is ongoing.

Petroleum derived synthetic detergents became popular in the United States during World War 2 due to shortages of animal and plant-derived fats and because they worked better when cleaning with hard water than traditional soap. By the 1950s, synthetic detergents were more commonly used than traditional soap in the United States. Many of the first synthetic detergents were made from compounds that contained branched carbon chains, which persist in the environment for far longer than their linear counterparts. Consequently, this led to the buildup of these foamy surfactants in water treatment plants as well as the formation of large flotillas of foam in waterways. Public pressure led the US and Europe to ban the use of alkyl benzene sulphonate (ABS) and other branched chain surfactants in 1965.

This sparked great interest in the development of synthetic detergents that biodegrade into environmentally friendly byproducts. Such interest has led to the development of the linear carbon chain compounds commonly used today, such as sodium lauryl sulfate and sodium laureth sulfate/ sodium lauryl ether sulfate (SLS/SLES). While these surfactants are still derived from petroleum, a nonrenewable resource, and have been shown to cause mild to moderate irritation of skin, they biodegrade significantly faster, and this has led to a drastic reduction in surfactant pollution of waterways. While the environmental friendliness of the biodegradation byproducts of the surfactants most commonly used today varies, the United States Environmental Protection Agency (EPA) monitors and regulates claims made by companies about the environmental friendliness and potential toxicity of the biodegradation byproducts of their cleansing products.

There have been continued efforts to develop surfactants that are milder to humans and pose less risk to the environment.  An emerging substitute for synthetic petroleum derived surfactants such as SDS are Alkyl polyglycosides (APGs). They are derived from plant based substances such as palm oil or wheat and exposure of APGs to skin and eyes is considerably safer than their petroleum derived counterparts. Studies have shown APG use, even in large quantities, pose no measurable environmental risk, while others report that more research is needed to confirm the true environmental impact of APGs. Although the use of APG surfactants currently have some disadvantages, such as the relatively high cost of production and uncertainties about the potential environmental impact of large scale use, further research into the development of APG surfactants shows a promising path to the creation of a naturally derived, non-toxic and environmentally friendly substitute for petroleum derived surfactants that is inexpensive, equally effective, and mass producible.

Another environmental issue with synthetic detergents is the addition of phosphates to these cleaning products. Phosphates are added to detergent as tripolyphosphate or as sodium/potassium phosphate. Phosphates interact with other ions in solution, like Calcium and Magnesium, to improve the washing ability of the detergent, especially when washing with hard water. Phosphates have also been shown to aid in killing germs when used in washing. However, most wastewater treatment processes generally remove only a small fraction of the phosphate in the water, and subsequently large quantities are released into waterways.

When large quantities of phosphates accumulate in waterways, it causes a bloom in algae and a subsequent lack of oxygen in the water, which severely damages the aquatic ecosystem. This process is called “eutrophication”. In 1959 detergents contained 7-12% phosphate by weight, by 1969 this increased to 15-17% by weight.  It is believed that during its peak use in the 1970s, half of all phosphates released by human activity was from detergents.