Buying natural products is a great first step to reducing the environmental impact of your personal care product use. Because naturally occurring chemicals have evolved with the environment, ecosystems often already have good ways of dealing with them. However, natural does not always mean better.
A lot of the natural chemicals that are used in personal care products have been extracted from where they usually occur and concentrated at much higher levels than they would otherwise be found. This can mean that the normal ways that the environment handles this chemical may not work as well.
A good example is d-limonene. This chemical occurs naturally in small amounts in citrus fruit peels. d-Limonene is also great at helping to remove sticky things. It is often extracted, concentrated and used in natural cleaning products. However, d-limonene is very toxic to aquatic life. In nature, those levels of d-limonene just do not exist where aquatic life does, so they have not evolved the appropriate detoxification mechanisms.
When using a natural chemical requires more resources than a synthetic chemical, the environmental impact can also be higher. Using a common synthetic chemical may be preferable to using a niche natural one, because there are economies of scale that help to reduce factors like the energy required during manufacture. Things other than the chemicals used can further contribute to environmental impact. For example, excessive packaging or transport can produce waste and add to greenhouse gas emissions.