The Daily Telegraph today reported that a “daily shower could be hazardous”. It said that US researchers have found that shower heads are “breeding grounds for bacteria and when water is passed through them, they blast out the bugs”. One of the main microbes identified was Mycobacterium avium , which can cause respiratory infections mainly in people with a weak immune system or chronic respiratory disease.
This small study looked at the microbes in 45 shower heads from nine cities in the US. Although it did find the presence of mycobacteria and other bacteria, it did not look at whether using a shower actually increased the risk of mycobacterial or other infections. This research should not cause healthy individuals to be concerned about having showers, as the mycobacterium identified is unlikely to cause illness in people with a healthy immune system.
Mycobacteria avium is common in the environment. Further research is needed to determine whether or not shower heads are an important source of mycobacterial infection in immunocompromised individuals.
Where did the story come from?
The research was carried out by Dr Leah M Feazel and colleagues from the University of Colorado. The study was funded by the Alfred P Sloan Foundation and the National Institute of Occupational Safety and Health. It was published in the peer-reviewed Proceedings of the National Academy of Sciences in the US .
What kind of scientific study was this?
This cross-sectional study looked at what bacteria can be found in shower heads. The researchers say that humans come into contact with microbes on a daily basis and shower heads may be one source of these. Layers of microbes can form on the inside of the shower head and spread as an aerosol that could be inhaled during shower use.
They say that the increasing use of showers instead of baths has been suggested as a potential cause for an increase in lung infections by a microbe called nontuberculous mycobacterium. The researchers suggest that the increase in immunocompromised individuals in the population means that identification of potential sources of infection is important.
The researchers investigated this by taking sample swabs from shower heads and looking for genetic material from different types of microbes. Swabs were taken of the insides of 45 shower heads from nine cities in the US. Samples were taken two or three times over a period of two to 12 months from some shower heads, to determine whether the types of microbes found changed over time. Samples of the water feeding into showers were also taken at 12 sites. The researchers also took samples of the aerosols created during a shower by running three different showers for 20 minutes unoccupied and then sampling the air and testing for microbes.
What were the results of the study?
The researchers found that the shower heads contained between two and 29 varieties of microbe, and the exact type varied between sites. The types of microbes found at each site tended to stay the same but the proportions fluctuated over time. The bacteria in the shower heads included bacteria that is found in water and soil.
Overall, the most common bacteria found in the shower heads were mycobacteria, mainly Mycobacteria gordonae and Mycobacteria avium . The latter can infect humans, mainly those who are immunocompromised (called an opportunistic infection), and those with HIV or AIDS are particularly at risk.
These mycobacteria were also found in the water samples, but were about 100 times more common in the shower head swabs. Mycobacteria avium was identified in one-in-five shower head samples, and accounted for an average of 32% of the microbes found in these samples.
As well as mycobacteria, there are other microbes that can be found in water and soil and are associated with respiratory disease in humans, such as staphylococcus and streptococcus bacteria, but these were less common in the samples. Very few of the samples contained the microbe that causes Legionnaire’s disease (Legionella pneumophila ), which accounted for only about 0.05% of the microbes identified.
When the researchers tested the aerosols created by running the showers, they found that the aerosols contained microbes representative of the water being fed into the shower, rather than the microbes living inside the shower head.
Mycobacteria were only identified in public water system-fed shower heads, and not in well-water-fed shower heads. The researchers thought this could be because mycobacteria are resistant to chlorine that is used to treat public water sources.
What interpretations did the researchers draw from these results?
The researchers conclude that, “shower heads may present a significant potential exposure to aerosolised microbes, including documented opportunistic pathogens”. They say, “the health risk associated with shower head microbiota needs investigation in persons with compromised immune or pulmonary systems”.
What does the NHS Knowledge Service make of this study?
These findings should not cause undue alarm as humans are constantly exposed to microbes. The authors of the study point out that indoor air usually has about a million bacteria per cubic metre, and tap water at least 10m bacteria per litre. Mycobacterium avium is one of several largely unavoidable bacteria known to occur in water, particularly in hot water supplies and aerosolised water, such as fountains. Many of these bacteria are not harmful to humans and our bodies’ defences are capable of protecting us from those that are harmful.
There is the possibility that the microbes identified could infect people whose immune systems are compromised but it is not clear from this study how common such shower-related infections might be. Further researcher will be needed to determine whether showers do increase the risk of non-tuberculous mycobacterial infection.
There are a number of points to note:
- Samples were taken from 45 shower heads in nine US cities. The results from this small sample may not be representative of all shower heads across the US or in other countries.
- Contrary to what is implied by some of the news reports, the Mycobacteria avium from the showerheads was not found in the aerosol produced by the shower, which would reduce the likelihood of it being inhaled. The researchers suggest that this may be because the mycobacteria are released in the very start of the shower and then diluted as the shower progresses. Further tests would be needed to determine if this is the case.
- The study did not test whether the microbes found would also be present in other water taps.