You can scrub a shower until the grout is white and still be living with a mold problem. In the showers we open up across Cape Cod, the worst growth is almost never on the surface. It is on the backer board, in the mortar bed under the floor tile, and on the framing — places that stay wet for years because the waterproofing behind the tile failed or was never there.
Nobody sees it, so nobody talks about it. But a growing body of research says what is behind the wall can matter for your, and your family's health, not just your bathroom. Here is a plain summary of what the science says, what is still debated, and what to do about it.
What is well established
This part is not controversial. The World Health Organization's 2009 Guidelines for Indoor Air Quality: Dampness and Mould and the U.S. Institute of Medicine's 2004 report Damp Indoor Spaces and Health both concluded that living in damp, moldy buildings are associated with:
- Upper respiratory symptoms (congestion, sore throat, sinus irritation)
- Coughing and wheezing
- Asthma symptoms in people who already have asthma
- Development of asthma, particularly in children
- Hypersensitivity pneumonitis in susceptible people
A 2011 meta-analysis in Environmental Health Perspectives (Mendell and colleagues) put numbers on it: visible dampness or mold in a home was linked to a 30–50% increase in a range of respiratory and asthma-related outcomes. The U.S. CDC and EPA both say the same thing in simpler terms — if you can see or smell mold, remove it and fix the water source.
The key phrase is fix the water source. Mold is a symptom. A shower with a failed pan is the disease.
The newer research: immune and neurological effects
This is where it gets more interesting, and more contested.
Dr. Ritchie Shoemaker is a physician who spent years documenting a pattern of illness in people exposed to water-damaged buildings, which he named Chronic Inflammatory Response Syndrome (CIRS). His core argument is that in genetically susceptible people (he points to certain HLA gene variants), the immune system fails to clear biotoxins from mold and bacteria in damp buildings, leading to ongoing inflammation that affects multiple systems — fatigue, cognitive problems, headaches, joint pain, and mood changes, alongside the respiratory symptoms.
Some of his published work:
- Shoemaker and House, Neurotoxicology and Teratology (2006), described changes in inflammatory markers and symptoms in people exposed to water-damaged buildings, and improvement after removal from exposure and treatment.
- Shoemaker, House and Ryan, Neurotoxicology and Teratology (2014), used volumetric MRI (NeuroQuant) to compare 17 patients with illness attributed to water-damaged buildings against controls, and reported measurable differences in several brain structures, which the authors attributed to chronic inflammation.
Independent of Shoemaker, a 2020 study from Dr. Cheryl Harding's lab at the City University of New York, published in Brain, Behavior, and Immunity, found that mice that inhaled mold spores — including spores with the toxins removed — showed innate immune activation in the brain, along with memory deficits and anxiety-like behavior. The authors' point was that mold's effects on the brain may not depend on "toxic" mold at all; the immune response to the spores themselves may be enough.
What is still debated — and why we're telling you
We install tile. We are not doctors, and we are not going to pretend this is settled.
CIRS is not a diagnosis recognized by major medical bodies, and the CDC's position is that while mold clearly causes respiratory and allergic problems, a causal link to the broader set of neurological and systemic symptoms has not been proven. Critics of Shoemaker's work point to small sample sizes and the lack of large, independent, controlled studies. Supporters point out that the pattern of symptoms is consistent across thousands of patients, and that the newer animal research is starting to show plausible mechanisms.
Our honest read: the respiratory and immune effects are proven. The neurological effects are plausible and under active study but not confirmed. Either way, the practical conclusion is the same — a wet shower assembly growing mold for years is not something anyone should be living with, and "it looks clean" is not evidence that it is.
If someone in your household has symptoms that get better when they're away from home for a few days and come back when they return, that is worth mentioning to your doctor, and worth having the bathroom checked.
Why showers are the usual source
Bathrooms have the most water in the house, and a tiled shower is the only place where water is deliberately thrown at a wall several times a day. If the waterproofing behind the tile is wrong, the mortar bed and backer board become a permanent sponge. The water never dries, the temperature is ideal, and there is plenty of paper-faced drywall and wood to feed on.
We see it most often with:
- Showers built with no membrane — tile set directly on drywall or on cement board with nothing behind it.
- Pan liners that were incorrectly installed, without a pre-slope underneath, so the mortar bed sits in a pool.
- Niches, benches, and curbs that were framed with wood and not properly waterproofed or had no pitch outwards and kept pooling water.
- Older Cape and Island homes where a bathroom was added or remodeled without a proper waterproofing system.
How to find out what's behind your tile
You don't always have to tear something out to get an answer. A proper inspection includes moisture meter readings on the tile, grout lines and surrounding drywall, thermal imaging to find wet zones behind the wall, a tap test for hollow tile, and experienced visual inspection at the drain and curb. In most cases that is enough to tell you whether you have a surface problem, a localized leak, or a shower that needs to be rebuilt.
If it does need to come out, the goal of the rebuild is simple: make sure this never happens again. Every custom shower we build uses a topical waterproofing system (the best system for water drainage) applied with our double-layer waterproofing method - a proprietary method that we used in over 1000 bathrooms without a single waterproofing failure.
Request a shower inspection or call (774) 368-9592. Batista Tile serves Cape Cod, Martha's Vineyard, Nantucket, Plymouth County, and the South Shore.
This article is for general information and is not medical advice. If you have health concerns, talk to your physician.
Not sure if your shower is leaking?
Our multi-factor leak test finds the answer in one visit without removing a tile: interview, photographed inspection, knock test, 22-point moisture map, thermal imaging and a bucket stress test, with a written diagnosis the same day. $299 per bathroom, credited toward the repair. Serving Cape Cod, the Islands and Plymouth County.