Common Questions with Dr. Pratt-Hyatt: Is There Mold in My Shower?

In our continuing series on frequently asked questions, our next blog post is about that constantly wet place in your home: The Shower. As many of you are aware, moisture is the cause of many environmental problems with homes.

Hello Again,

In our continuing series on frequently asked questions, our next blog post is about that constantly wet place in your home:  The Shower.  As many of you are aware, moisture is the cause of many environmental problems with homes.  As I stated in the past, one important number is the water activity (aw) ( aw x 100 = % relative humidity at equilibrium) and the longer a materials’ aw is over 0.75 the greater the risk for fungal growth1.  We have a room that has a lot of water and humidity.  First, we need to make sure the water stays in the shower.  Is the shower properly sealed, is the doors/shower curtains keeping water where it should, is there proper ventilation in the bathroom?  These will help keep the problems contained to just the shower area.  Next, we will discuss two items.  What microorganisms are potentially living in your shower and second what can be done about it.

The organisms that could be invading your shower fall into two main categories which are bacteria and mold.  The most prevalent species of bacteria belong to the Actinobacteria, Alphaproteobacteria, Bacteroidetes, and non-tuberculous Mycobacteria.  In a recent paper from Moat et al., they looked at samples from four different houses.  They found that a majority of the bacteria found was from the Alpha Proteobacteria family (Figure 1)2.  In another study by Adams et al., they look at microbes on both kitchens as well as bathroom surfaces (Figure 2) 3.

Figure 1: Bacteria families found in showers of different houses.

Figure 2: Bathroom Microbe Sampling Chart

Regarding toxic mold, we are lucky that there isn’t much cellulose found in the shower. As mentioned in my earlier blog on home remediation, mold needs cellulose to live.  Cellulose is often found in wood, fiberboard, dust, gypsum, and paper.  This is why it is important to keep water in the shower and humidity down in the bathroom.  However, there other types of mold, which can be in a variety of colors, that are able to grow on the molding, caulk, or shower curtain.  These types might not produce toxins but could cause health issues through histamine reactions.  Here are my types to prevent mold in the shower:

  1. Practice good ventilation.  Turn the exhaust fan on when showering and leave it on for 30 minutes afterward.  Keep the bathroom window open when showering.
  2. Use a shower curtain and replace it often. Make sure your curtain or plastic liner is tucked into the bathtub.  Plastic liners that show signs of mold growth should be replaced.
  3. Treat biofilm in the drains. One source of cellulose for mold could be body hair, oil, and soap debris that accumulates in the shower drain.  Using a device to pull out hair caught in the drain and pouring vinegar down the drain weekly could help prevent spores from forming.
  4. Final spray down: After showering wipe down the entire shower including your soap bottle and shampoo bottle.
  5. No suction cup non-slip mats. Mold and bacteria love to grow on these.  They are difficult to clean and trap water.

I hope this helps.  If you have any questions feel free to contact us

Resource List

  1. Nielsen, K.F., Holm, G., Uttrup, L.P. & Nielsen, P.A. Mould growth on building materials under low water activities. Influence of humidity and temperature on fungal growth and secondary metabolism. International Biodeterioration & Biodegradation 54, 325-336 (2004).
  2. Moat, J., Rizoulis, A., Fox, G. & Upton, M. Domestic shower hose biofilms contain fungal species capable of causing opportunistic infection. J Water Health 14, 727-737 (2016).
  3. Adams, R.I. et al. Microbes and associated soluble and volatile chemicals on periodically wet household surfaces. Microbiome 5, 128 (2017).

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