There is little that is more relaxing than soaking in a hot tub – except, perhaps, soaking in an herb-infused hot tub. Health clubs that feature herbal spas are becoming big business throughout the world.
Not only do bathers find the different scents soothing, intoxicating or exhilarating, but proponents also use herbal baths to treat stress, pain and muscle soreness, as well as a variety of skin conditions such as acne, dermatitis, eczema, scalp itching, flaking and dandruff.
However, according to a 2018 article appearing in Nature Magazine, because spa water is often chlorinated to guarantee hygiene, chemical reactions may occur between components of the herbs and chlorine resulting in questionable by-products.
Commercial health spas commonly add ginger, ginseng, rose, lavender, green tea, mint and rosemary to improve bathers’ experience.
According to the authors of the Nature article, the oils of ginger contain many components that are used in healing therapies for health problems such as skin diseases, joint pain, hypertension, insomnia or diarrhea. Ginseng was used in ancient times as an aphrodisiac, an anti-aging agent and an energizer. Ginseng is also currently used to enhance energy levels and as an antioxidant. Mint and rosemary have also been used for medical purposes. Mint has been used as a digestive aid and as a treatment for colds, cough, fever and diarrhea. A rosemary bath is believed to benefit mental and physical health; this aromatic plant is one of the most widely used herbal medicines for muscle pain.
But it is still important for such commercial spas to manage the disinfection properties of the water to prevent microbial pathogens. So even as the herbs are added to the water, so too is chlorine to protect bather health.
And while there are many studies that have examined disinfection by-products in spas and swimming pools, few have analyzed the role of natural organic matter derived from herbs.
To that end, the authors collected water samples from a spa to investigate the formation of disinfection byproducts, specifically trihalomethanes, through herbal and disinfection addition experiments.
The experiments showed that the organic molecules introduced by the herbs are significant precursors to the formation of trihalomethanes in spa water.
This is an important finding because while herb-treated water may provide health benefits, trihalomethanes and other disinfection by-products are commonly regulated carcinogens.
Pool and spa water chemistry is complex, and the exposure pathways to such disinfection by-products are still relatively unknown, so the health risks are largely uncertain.
That said, some studies have linked exposure to chlorinated disinfection byproducts to asthma, as well as bladder and colorectal cancer.
When bathers enter chlorinated pool and spa water, they inadvertently provide the precursors to some of these disinfection by-products through body oils, sweat, urine and other organic materials. Part of the impetus in encouraging bathers to shower prior to entering recreational water is to wash some of this matter away to limit the formation of such potentially harmful disinfection by-products.
Chloroform (a trihalomethane), is one such common by-product, and it has been shown that consumption of water containing high concentrations of trihalomethanes may lead to liver, kidney and central nervous system problems.
Furthermore, it has been found that trihalomethanes are absorbed through the skin and through respiration. As such, the World Health Organization recommends a maximum concentration of trihalomethanes of 100 ug/L for all types of pools.
However, a 1990 study of 101 swimming pools in Miami, Florida, reported an average concentration of 125 ug/L of trihalomethanes (mainly chloroform).
With herbal spas becoming increasingly popular around the world, it is relevant to examine their role in the formation of disinfection by-products.
The authors of the Nature article simulated herb treated spa water using water samples containing ginger, ginseng, mint, rosemary and various chlorination levels normally found in spa water. The control group was chlorinated spa water without the addition of herbs. The difference between trihalomethane formation in the herb-containing sample and the control sample (no herbs) was significant in all cases except for the mint sample. Ginseng and rosemary showed the highest trihalomethane levels which significantly exceeded the maximum concentration established by the World Health Organization.
The authors state: “The potential to generate high levels of chlorinated byproducts on a daily basis in the herbal spa community could represent an environmental and health assessment problem because long-term exposure to high THM (trihalomethane) levels (>50 μg/L) could result in serious health problems, such as bladder cancer.”
They also noted that increased temperature accelerates the formation of such by-products, and that higher concentrations of herbs and chlorine resulted in higher levels of disinfection by-products, with chloroform being the most prevalent.
They recommend some general control measures be taken, especially because spa bathers are typically positioned with their faces at the airwater interface, where the volatile disinfection by-products accumulate in high concentrations.
Such measures include pre-swim showering, an efficient filtration system, regular water replacement and improved ventilation.