Brain Fog: Common Causes and When Mould Could Be Behind It

Brain fog is a symptom, not a diagnosis. Thinking feels slow and cloudy. Words and names slip away, and it is hard to concentrate or follow a conversation.

It has many possible causes. The most common are poor sleep, thyroid problems, iron or vitamin B12 deficiency, depression, long COVID and the menopause transition. There is no single test for brain fog itself, but a doctor can check most of these causes with ordinary blood tests and questions.

If those have been ruled out and your thinking is clearly worse in one particular building, consider exposure to a water-damaged building and CIRS (Chronic Inflammatory Response Syndrome).

What is brain fog?

“Brain fog” is a word patients use, not a medical term. Researchers asked young people with postural tachycardia syndrome (POTS) to describe it. In POTS, the heart races when you stand up. The most common words were “forgetful”, “cloudy” and “difficulty focusing, thinking and communicating” [1].

So brain fog is a cognitive complaint. Cognitive means to do with thinking: memory, attention and how fast you think.

Typical signs include:

• losing your train of thought mid-sentence, or forgetting why you walked into a room
• struggling to find words or names
• reading the same paragraph several times
• feeling mentally exhausted after tasks that used to be easy
• finding it hard to do two things at once

What are the most common causes of brain fog?

Well studied The causes below are common, well studied and mostly easy to test for. Go through them with your doctor before looking for rarer explanations. More than one can be present at the same time.

Cause	How a doctor checks it
Poor sleep	Questions about your sleep and sleep disorders
Thyroid problems	TSH blood test, often with free T4
Iron or vitamin B12 deficiency	Blood tests
Depression and anxiety	Talking about your mood
Long COVID	Your history: did it start after an infection?
Menopause	Talking with a healthcare professional

Poor sleep

Even short-term sleep loss clearly harms attention, working memory (holding things in mind for a moment) and how fast you process information. A meta-analysis combines the results of many studies. In a meta-analysis of experimental studies, lapses of attention increased the most [2].

Too little sleep, broken sleep, shift work and untreated sleep disorders are among the most common reasons for foggy thinking. Sleep problems are also common in CIRS. Read more in CIRS and sleep.

Thyroid problems

A clearly underactive thyroid (overt hypothyroidism) harms memory and thinking and lowers mood. These problems largely improve with treatment. In mild (subclinical) hypothyroidism the effects are small [3].

A simple blood test (TSH, often with free T4) shows whether the thyroid is the cause.

Iron or vitamin B12 deficiency

Iron: Iron deficiency has been linked to poorer attention and other thinking problems, even before it causes anaemia. Most of this research has been done in children [4].

Vitamin B12: B12 deficiency is linked to problems with thinking. A vegetarian or vegan diet and long-term use of the diabetes medicine metformin can lower B12 levels. B12 treatment improves thinking only in people who are actually deficient [5].

Both are checked with blood tests. Do not start high-dose iron on your own. Too much iron is harmful, so agree on testing and supplements with your doctor.

Depression and anxiety

Depression does not only affect mood. In a meta-analysis, people with depression had moderate problems with planning and organising (executive function), memory and attention. Problems with attention and executive function lasted even after the low mood had lifted [6].

Anxiety and long-term stress can also make it hard to think clearly.

Long COVID and other illness after infection

Brain fog is one of the most common symptoms after COVID-19. In a meta-analysis, about 22% of people (about 1 in 5) had problems with thinking 12 weeks or more after the infection. About 32% (about 1 in 3) had fatigue [7].

Brain fog is also typical of ME/CFS (chronic fatigue syndrome), which often begins after an infection. See CIRS vs ME/CFS.

Menopause

Many women notice forgetfulness and difficulty concentrating during the menopause transition. A white paper from the International Menopause Society says these changes are common. It encourages women to discuss them openly with a healthcare professional. The paper also reviews the role of oestrogen and of menopausal symptoms such as hot flushes and poor sleep [8].

Other causes worth asking about

These can also play a part:

• medicines, for example some sleeping pills and other sedating drugs
• alcohol
• blood sugar problems
• coeliac disease
• dehydration
• POTS and other medical conditions

Bring a list of everything you take to your appointment. Ask whether any of it could affect your thinking.

Can mould cause brain fog?

It is possible. But the evidence is weaker than for the causes above, and it is honest to say so.

• Reviews of mould-related illness describe symptoms in many organ systems, including the brain and nerves, in people exposed to moulds and their toxins [9].
• Small patient studies: One small study In one study of 31 mould-exposed patients, most had problems in several areas of memory and thinking [10]. There was no unexposed comparison group with the same complaints. So the study cannot prove that mould was the cause.
• Animal research: Preliminary research Mice repeatedly breathed in spores of the mould Stachybotrys, even spores with the toxins removed. This switched on the brain’s immune response and caused memory problems and anxiety-like behaviour [11]. Read more in CIRS and neuroinflammation.
• Official reviews are cautious. In 2004, the US Institute of Medicine found too little evidence to decide whether damp indoor environments cause neuropsychiatric symptoms (symptoms of the brain and mind) in people [12]. That means the question is open. It does not mean the link has been disproved.

How does the CIRS model explain brain fog?

Expert view Dr Ritchie Shoemaker describes CIRS as a chain of events [13]. HLA-DR genes help the immune system recognise threats.

This model comes mainly from Dr Shoemaker’s own research group. Independent researchers have not widely confirmed it.

When should you consider mould and CIRS?

Mould exposure is worth looking into if your brain fog comes with several of these:

• A building pattern: your thinking is worse at home, at work or at school and clears up when you are away for several days. In Dr Shoemaker’s studies, symptoms returned within days of going back into the water-damaged building [13].
• Known or suspected water damage: a musty smell, past leaks, visible mould or other signs of water damage.
• Symptoms in many systems at once: for example fatigue, headaches, muscle aches, sensitivity to light, numbness or tingling, and sinus or breathing problems alongside the brain fog. In Dr Shoemaker’s study, participants had on average 23 of the 37 symptoms assessed, well over half [13].
• Others affected: other people in the same building have symptoms too.
• Ordinary causes ruled out: blood tests and other check-ups have not explained the problem.

None of these signs proves CIRS. They mean that the indoor environment deserves the same attention as the other possible causes.

	Common causes (sleep, thyroid, iron/B12, depression, menopause)	Mould exposure and CIRS
Pattern	Usually the same wherever you are	Often worse in a particular building, better away from it
Other symptoms	Depends on the cause, often limited to a few systems	Many systems at once: fatigue, pain, sinuses, light sensitivity
How it is checked	Ordinary blood tests, sleep history, mood assessment	Exposure history, VCS test, CIRS-specific blood tests
Strength of evidence	Strong, from large studies and meta-analyses	Limited; mostly small studies and one research group’s model

Is there a test for brain fog?

There is no single blood test for brain fog itself. Your doctor can check memory and concentration with short thinking tests and look for causes with blood tests.

If mould is suspected, the visual contrast sensitivity (VCS) test is a simple and cheap first screen. You can take it online at home. It measures how well you can see faint grey stripes.

The link between biotoxins and contrast sensitivity was first described in people who had fallen ill from toxic algae [14]. In a study by Dr Shoemaker’s group, VCS was abnormal in all 26 people who had fallen ill in water-damaged buildings. It improved during treatment [13].

Keep its limits in mind:

• It is not specific. Eye conditions and other illnesses can also lower contrast sensitivity [15]. An abnormal result does not mean you have CIRS.
• A normal result does not rule CIRS out.
• It cannot explain brain fog on its own. It is one piece of the assessment, not a diagnosis.

What should you do next?

1. See your GP first. Describe your brain fog in concrete terms: what you can no longer do, since when, and whether it changes with place or time of day. Ask for basic tests: blood count, TSH, iron stores (ferritin), vitamin B12 and blood sugar. Also discuss sleep, mood and your medicines.
2. Keep a symptom diary for a few weeks. See the box below. A building pattern is one of the most useful clues.
3. If the building pattern is there, take a VCS test. Look for signs of water damage in your home or workplace.
4. Investigate and fix the building. Reducing exposure is the first step, whatever the diagnosis turns out to be. The water damage guide explains how to proceed.
5. Find a doctor who knows CIRS if the VCS result is abnormal or the pattern is clear. How CIRS is diagnosed explains the blood tests (such as HLA-DR, C4a, TGF-β1 and MSH) and why no single result is enough.

How strong is the evidence?

• Well studied The links between brain fog and poor sleep, thyroid disease, iron and B12 deficiency, depression and long COVID rest on large studies and meta-analyses.
• Preliminary research The link to mould rests on patient reports, small clinical studies and animal research. Large independent studies in humans are missing, and official reviews have called the evidence insufficient [12].
• Expert view The CIRS model and the use of the VCS test in it come mainly from Dr Shoemaker’s research group.

That is why the sensible order is to rule out the common causes first, without forgetting the building you spend your days in.

Remember these

Check yourself: Can you explain why an abnormal VCS test does not prove CIRS?

What next?

• Read about the VCS test.
• Learn how CIRS is diagnosed.
• If tiredness is your main problem, read unexplained chronic fatigue.

References

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3. Samuels MH. Psychiatric and cognitive manifestations of hypothyroidism. Curr Opin Endocrinol Diabetes Obes. 2014;21(5):377–383. PubMed
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12. Institute of Medicine. Damp Indoor Spaces and Health. Washington, DC: National Academies Press; 2004. doi:10.17226/11011
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14. Hudnell HK. Chronic biotoxin-associated illness: multiple-system symptoms, a vision deficit, and effective treatment. Neurotoxicol Teratol. 2005;27(5):733–743. PubMed
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