Q: My hair has been dry and brittle lately. How can I take better care of it? And what can my hair tell me about my health?

A: You may not know this, but your scalp has its own microbiome. It’s a special part of the microbiome that exists elsewhere all over your skin, contending with a high density of hair follicles and their oil-producing sebaceous glands.

The scalp microbiome contains many common bacteria but is also home to a number of fungi, like the Malassezia species that is associated with seborrheic dermatitis, an inflammatory condition of the scalp causing red, itchy skin and dandruff. Which microbes inhabit your scalp is influenced by your age, sex, hair texture, the environment – including moisture, UV exposure and air pollution – and your behaviors, such as bleaching or coloring your hair or wearing a head covering.

Most dermatologists agree that the first step toward healthy hair is a healthy scalp, and how you care for it can affect the health of your hair at every age. This means using “gentle” or “sulfate-free” hair care products that you massage onto your head – not scrub into a voluminous lather – with warm instead of scalding water. For many people, it also means not washing your hair every day.

The right frequency to wash your hair depends on the individual – washing daily can strip away the skin’s oils leading to brittle hair or an overly dry scalp (for women, sebaceous gland production drops after menopause, so what worked in your 30s may not be what works best in your 60s). The American Academy of Dermatology recommends washing once weekly or every other week for African Americans.

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. 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.

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 gray 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 gray” 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 color.

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.