The Bacteria Inside Your Shower Head: What Research Actually Shows

Most people assume their shower is clean because the water comes from the mains. We used to hear this a lot when we were working in hospital infection control. The logic seems reasonable: treated water, regular use, surely it rinses itself out.

The research tells a different story, and I think it's important that more people understand what it actually shows.

The founding study

In 2009, a team of researchers at the University of Colorado published what became a landmark paper in the Proceedings of the National Academy of Sciences. They sampled shower heads from households across the United States and analysed the bacterial communities living inside them.

The headline finding was stark. Opportunistic pathogens, bacteria with the potential to cause illness in humans, were present in showerhead biofilms at concentrations more than 100 times higher than in the tap water supplying the same showers. The shower head wasn't simply a conduit for the water. It was actively concentrating and culturing bacteria.

Mycobacterium avium, a bacteria associated with lung disease, lymph node infections and other serious conditions, was found in a significant proportion of the samples. The researchers noted that its presence was particularly concerning given how directly it can enter the respiratory system during showering.

The 2018 follow up

Nearly a decade later, the same research group published a much larger study in the journal mBio, this time covering households across both the United States and Europe. The findings confirmed and expanded the original work.

They found that showerhead bacteria vary by geography, water chemistry and source, but that Mycobacterium species were near universally present. The study also identified a striking pattern: the geographic regions in the US with the highest concentrations of potentially pathogenic mycobacteria in shower heads were the same regions where nontuberculous mycobacterial lung disease was most prevalent in the general population. The correlation was significant enough for the researchers to propose a direct link.

How quickly bacteria builds up

Separate research tracking biofilm development over time produced findings that I find particularly useful for understanding the practical risk. Legionella pneumophila, the bacterium responsible for Legionnaires' disease, appeared in shower head biofilms within four weeks of installation. After 22 weeks, it was present in large numbers. Mycobacterium species reached large numbers after 30 weeks.

This means that a shower head used for six months or more has very likely built up a significant bacterial population, regardless of how often the bathroom is cleaned.

The aerosol problem

What makes shower head bacteria uniquely concerning compared to, say, bacteria on a kitchen surface, is the delivery mechanism. When a shower runs, it generates aerosols, particles so fine they remain airborne and are inhaled deep into the lungs rather than filtered out in the upper airways.

Research has consistently shown that inhalation of water associated aerosols during showering is the primary route through which shower head pathogens reach the human body. A contaminated surface you touch is relatively easy to manage. A contaminated aerosol you breathe in every morning, for years, is much harder to avoid without addressing the source.

The antimicrobial shower head question

A 2023 study published in Frontiers in Microbiomes specifically looked at whether shower heads marketed as antimicrobial reduced bacterial exposure during showering. The findings were that most commercially available antimicrobial shower heads produced water quality similar to standard shower heads under real world household conditions.

The approach we use in the Fereno RefreshFlip works differently. Rather than a coating, the antibacterial action is built into the material and the mechanism itself, which is the same principle we applied in clinical settings for over three decades.

Where to find the research

If you want to read the studies referenced in this article, they're freely available. The original 2009 PNAS paper, the 2018 mBio study, and the 2023 Frontiers research are all publicly accessible. I'd encourage anyone interested to read them. The science is clear, accessible and worth understanding.

Steve Bennett, Co Founder, Fereno