In their lifecycle analysis of bar soap versus liquid soap, researchers Annette Koehler and Caroline Wildbolz conducted an “in-depth life cycle assessment of nine home-care and personal hygiene products.” Their goal was to quantify and compare the environmental impacts of products that could be used for the same application – for example, bar versus liquid soap for hand washing.
In this work, they included the impacts related to the production and delivery of each type of soap as well as the actual use of the product (for, in this case, hand washing). In this work, Koehler and Wildbolz quoted previous studies from industry that looked at our hand washing tendencies. According to this research, we use a lot more liquid soap compared to bar soap each time we wash our hands. But, when we use bar soap, it takes us about 42% more water to get the job done.
According to researchers at the Swiss Federal Institute of Technology in Zurich, bar soap requires one-fifth of the energy to produce compared to liquid soap.
In conducting their life-cycle assessment, these researchers found that energy requirements for “raw chemical production and supply” and “packaging production and supply” were responsible for the high levels of energy use for liquid versus bar soap. They found that liquid soap is typically produced using a variety of different chemicals (meaning that the liquid is, technically speaking, usually a detergent and not actually soap per se). Furthermore, this liquid can use up to 20 times more packaging than bar soap and contains significantly more water, making it heavier and less efficient to transport from where it’s produced to where it’s used.
Now for the caveats:
- not all liquid soaps are detergents (for example, Dr. Bronner’s castile liquid soap) and some bar “soaps” should actually be called detergents and are full of chemicals.
- while bar soaps are better from an energy standpoint, as mentioned above, Koehler and Wildbolz found that the bar has a larger impact on land use than its liquid counterparts due to its use of vegetable oils (which are farmed).
So to consclude : There’s a clear energy winner when it comes to washing your hands—bar soap
To read the complete study, including its discussion of liquid versus power detergents for your clothes (spoiler: stick to liquid) and toilet bowl cleaners, you can see:
Koehler, A. and C. Wildbolz. Environmental Science & Technology. Published 2009 doi:10.1021/es901236f (online at : http://www.ncbi.nlm.nih.gov/pubmed/20028065)