On the other hand, you don’t want to go too long without washing, as the oil from the scalp’s sebaceous glands can eventually irritate the skin. To give an extreme example: In 2015, scientists looked at a group of Antarctic geological investigators who did not bathe for three months. After the expedition, Malassezia fungi on the geologists’ scalp microbiomes increased about sevenfold, and this correlated with itching and dandruff.

All these conditions that impact your scalp microbiome and hair health change over time. So if you’re not thrilled with how your hair feels, try switching up your washing routine – even if the habit was ingrained since childhood – as an experiment for a few weeks. You might surprise yourself what a difference it makes.

What can my hair tell me about my health?

Hair loss or thinning

As we age, our hair becomes thinner and more brittle, and both men and women start to bald, typically in different patterns, around the scalp. Though these changes are normal – due to alterations in the stem cells of our hair follicles and hormonal shifts such as with menopause – they can feel distressing because of how deeply they can impact our perception of ourselves.

Men tend to start to notice balding or hair thinning as early as their 20s. By their 50s, about half have experienced hair loss. Symptoms in women can show up a bit later – by age 70, half of women notice some degree of hair thinning.

Topical minoxidil has been shown to be effective in male and female pattern hair loss, also known as androgenetic alopecia, but you need to give it about six months to see possible results. Small studies have found oral minoxidil, which may be prescribed off-label for hair loss, to be as effective as topical minoxidil by some metrics, but not by all. Larger studies are still needed to understand who might best benefit from these therapies.

Other causes of hair loss are autoimmune conditions, such as alopecia areata, or drugs. Drug-induced hair loss is almost always reversible because this tends to only affect hair in the “growing” phase. In the case of chemotherapy, these drugs target rapidly dividing cells often seen in a tumour – and our hair gets caught in the line of fire because they are a part of the body where our cells are normally rapidly dividing. How we style our hair is also linked to hair loss: Wearing a tight ponytail or using a hair straightener or curling iron are all culprits.

Lastly, some infections can cause our hair to fall out – patchy hair loss, for example, could be a sign of a fungal infection of the scalp. When in doubt, snap a picture and send it to your health-care provider.

Coarse, dry hair

Thyroid disease – both hyperthyroidism and hypothyroidism – can lead to changes in hair texture, increased breakage and hair loss. This is usually accompanied by other signs of thyroid dysfunction that can include changes in bowel habits and even heart rate or blood pressure. Nutrient deficiencies – such as from vitamin D, iron or folate – may be linked to hair abnormalities, but these should be evaluated and diagnosed by a physician before taking unnecessary supplements. (While we’re on this subject, I’ll answer a common query: No, there’s no evidence that taking prenatal vitamins when you’re not pregnant will help your hair grow faster.)

Premature grey or white hair

Certain stem cells in our hair follicles contain melanin that provide pigment to our hair – but many people lose these stem cells with age. If and how fast we “go grey” is largely determined by our genetics, but smoking and alcohol use have also been shown to speed up the rate at which our hair loses colour.

Stress also appears to play a big role in greying hair. A study published in Nature in 2020 found that activation of the sympathetic nervous system – commonly associated with the “fight-or-flight” response – depletes the melanin-producing stem cells in our hair follicles.

But how quickly stress can impact your hair has become a bit mythologised. For instance, an old legend claims that sudden psychological stress turned Marie Antoinette’s hair white the night before she encountered the guillotine during the French Revolution. However, this is unlikely to have occurred. Depletion of melanin, even if rapid, will affect new hairs that will grow from the follicle – it would not transform the visible hairs already protruding out of the scalp. The greying hairs that we observe among American presidents over their four-year terms is a more realistic timeline.

How much hair loss is normal?

Humans have about 5 million hair follicles on our bodies. While 80 to 90% of our hair is in the “growing” phase of their life cycle at any one moment – a phase that can last about two to six years – each hair eventually enters a shedding period at the end of the cycle. It may sound wild, but because of this, it’s normal to lose around 100-200 hairs per day from the scalp. Our bathroom drains can attest – help your plumber and get drain protectors.

What I want my patients to know

Much of the time, changes in hair are nothing scary. But if your hair starts falling out in clumps, or the skin underneath the hair appears red and irritated, talk to your physician. Excessive hair loss may be a sign of telogen effluvium, a condition in which at least half of the scalp hair transitions out of the “growing” phase and becomes easily pulled from the scalp. Telogen effluvium can happen when the body experiences major stressors such as after a surgery or if you’re malnourished.

Trisha Pasricha is an instructor of medicine at Harvard Medical School.