Ask your five closest friends how often they wash their hair and you're almost certain to get five different answers. Every day. Every other day. Once a week. Every other week. It depends.

So ... who's right and who's wrong?

"Hair washing is a complicated topic, and there is definitely not a one-size-fits-all recommendation for how frequently a person should wash theirs," says Dr. Jennifer Maender, a dermatologist at Houston Methodist. "The type of hair you have, how old you are, the styling practices you utilize — there are many factors that come into play to answer this question."

Personal preferences aside, Dr. Maender is here to explain what makes for healthy hair, when you might be overwashing or underwashing your hair and how to strike the right balance.

Hair care 101

The longer between washes, the greasier hair becomes. It's why our physical appearance ends up being the main reason we keep up with our hair-washing routines. But while a greasy head of hair may be taboo in today's world, it isn't a bad thing from a health perspective.

"Sebum, or oil, serves an important role for our hair by moisturizing it," explains Dr. Maender. "This makes hair less prone to breakage and contributes to making it healthy and shiny looking."

Glands located all over the scalp, called sebaceous glands, naturally produce this oil into hair follicles. As it travels up each hair shaft, the oil coats hair strands and keeps them from becoming dry and brittle. Hair gets greasy as more and more oil is distributed throughout, but how quickly this happens varies from person to person.

What makes our hair "dirty" and in need of cleaning? Dirt, dust, pollen and environmental pollutants can get trapped in our hair, as do the products we use to style it. Oil can also build up to amounts that become irritating to the scalp. This is what dermatologists call seborrheic dermatitis or dandruff, which makes the scalp itchy, red and flaky.

"Shampoo acts as an emulsifier to trap oil and dirt — rinsing them from our scalp and rendering our hair 'clean,'" says Dr. Maender.

How often should you wash your hair?

When it comes to hair-washing frequency, Dr. Maender says most people should wash their hair two to three times per week. Several factors increase and decrease this range, however.

"People with thick, curly, or long hair should shampoo less frequently, as their hair type tends to not distribute oil as easily down the shaft of the hair," explains Dr. Maender. "Because African American people have a hair type prone to dryness and breakage, it is necessary to shampoo far less frequently, such as once a week — or even up to once every two weeks if their scalp does not become irritated or itchy."

Age and climate are also considerations. Children and elderly people produce less oil and will need to shampoo less frequently, and someone living in a dry climate can get away with less frequent hair washing than a person who lives in a humid environment, like Houston.

Then there are those who may need to wash their hair more frequently than twice a week.

"People with fine, thin hair will have oil distributed over the hair shaft more quickly and need to shampoo more often," says Dr. Maender. "As will people who sweat frequently or wear hats and helmets."

Is it bad to wash your hair every day?

There are many reasons you might want to wash your hair every single day, ranging from exercising frequently and working outdoors to having naturally greasy hair or just personal preference. But given the important role oil plays in moisturizing hair, could a daily wash be doing more harm than good?

It depends.

Overwashing your hair can lead to dryness and hair breakage, but this doesn't happen with a daily wash for everyone — particularly if you have fine, thin hair.

People who wash their hair more frequently than their hair type requires can help decrease the chance of dryness by:

• Shampooing mainly at the roots and scalp and not distributing the shampoo through to the ends of the hair
• Using conditioner after shampooing
• Using dry shampoo products between washings if you have fine, thin hair
• Washing with water only or using a conditioning cleanser, also known as co-washing or co-poo

Does the shampoo you use matter?

"This is a much harder question to answer because there is so much variability in the market," says Dr. Maender. "Some hair products suit certain hair types better than others, but there's no one perfect shampoo for everyone."

She adds that some people, such as those who suffer from dandruff, will even need medicated shampoos.

Your dermatologist can help you understand your hair type and offer specific recommendations on how to keep your hair looking healthy and shiny.