The answer to this question is simple and rather unsatisfying: it depends! As long as it's not pouring down rain the moment you walk out of the salon, blowouts can last anywhere from a couple days to a full week.
So why is there such a big range? This is because of several reasons:
No two people have exactly the same hair - some people have hair that holds a curl super well, and other people have hair that will flop back into its old shape within a few hours (unless you cover it with a whole heck of a lot of hairspray). If your hair is sparse, fine, damaged, and/or straight, it'll struggle to hold the shape of the blowout for more than a couple days.
If you get a blowout that resembles your normal wave/curl pattern, it'll last longer than a blowout that looks nothing like your normal hair. Straight hair struggles to hold a voluminous blowout while curly hair struggles to hold a straight, sleek blowout. If you want your blowout to last, go with something that's not too drastically different from the way your hair normally shapes itself.
Humidity can have a huge impact on the longevity of a blowout. Winter blowouts tend to last longer than summer blowouts for this very reason - there's significantly less humidity. Most folks have some sort of wave or curl pattern to their hair, and summer humidity accentuates that pattern. A blowout on a hot, humid day may only last a few days before the hair reverts to its natural pattern.
If you're a gym rat, we've got some bad news. Blowouts and sweaty workouts are, unfortunately, not very compatible. Sweat dampens the hair and spreads sebum from the scalp, making it appear greasy. Sweat also causes baby hairs to return to their natural curl/wave pattern, leaving you with unsightly flyaways. Positioning a fan straight at your face while you work out can be a little helpful for keeping cool as you work out, but nothing can stop the fact that when your blood gets pumping your body will get sweaty.
Sebum is your body's built-in moisturizing system. When you wash your hair, you remove most of the sebum from your scalp. Your scalp doesn't really like this - In response, your scalp increases sebum production to regain its preferred balance of oil at the roots.
Every scalp generates sebum at a different rate, and every scalp has a different amount of sebum that it considers to be an acceptable amount. If you have a scalp that produces sebum slowly, you'll be able to make your blowout last longer than if you have a scalp that produces sebum quickly. The same thing is true if your scalp tolerates a lower amount of oil at the root - once it gets to the amount of sebum that it deems acceptable, it'll slow down sebum production. And if your blowout still looks good at that point, you can make it last a smidge longer.
Even if your scalp produces a lot of sebum, your hair might not show it. This is particularly true for thick, textured and/or porous hair - these hair types are more likely to absorb sebum, so it doesn't just sit on the surface of the hair. Thus, the scalp can produce a good bit of sebum over several days without really throwing off the appearance of the blowout.
If you have fine, sparse, straight, or low-porosity hair, the opposite is true. These hair types struggle to absorb or hide sebum, and are easily weighed down.
Some scalps don't do well when they go long periods of time without being washed. Even if they don't look greasy, they can get smelly and itchy. This is typically due to the microflora (the microscopic organisms that live on your scalp) becoming unbalanced, which can happen when the scalp isn't cleansed regularly with an appropriate product. So even if your blowout holds its shape perfectly, it won't be worth it if your scalp is itchy and uncomfortable. Going long periods of time without washing when you have this issue can also worsen the problem by increasing the likelihood that you experience a serious infection.
So now that you know all the things that can shorten the life of a blowout, what can you do to lengthen it?
If your hair is oily at the scalp, cover it up with a headband. Go for a one that'll cover a good bit of surface area - big, wide headbands are your friends. If your hair is super oily, pair your headband with a high ponytail - this will obscure more of your scalp than a low pony, and the pony will still allow you to show off what remains of your blowout.
If your blow out has lost its shape or you're getting a little too oily at the scalp, convert it to a sleek ponytail. When you put it up, be careful not to mess too much with your roots - by combing or bushing them, you'll move the oil from your scalp and make your hair appear greasier.
Don't get us wrong - we think dry shampoo is super useful. But in order to prevent the risk of infection to your scalp, it's essential to not rely on dry shampoo alone. Excessive amounts of dry shampoo in combination with high amounts of sebum can create an overly hospitable environment for yeast, the overgrowth of which can lead to an itchy and smelly scalp.
So don't be too eager to whip out your dry shampoo - use as little of it as you can. Invest in a high quality, lightweight dry shampoo, and definitely don't use any questionable homemade versions that could further increase the presence of harmful microbes (coco powder, we're looking at you!)
When you're using dry shampoo, follow some basic best practices. Apply it directly to the most oily parts of your hair, instead of all over. If your updo has fallen a little flat, flip your hair over and apply it to the base of your scull. Let it sit for a few minutes, and then blast your hair with cool air via hair dryer. Be sure to point the air away from your roots and towards your ends, as this will better air out your scalp.
This won't prolong the life of your current blowout, but switching to sulfate-free shampoo could prolong the life of future blowouts. Sulfate free shampoos can be helpful in re-training the scalp to produce less oil. Less oil is stripped from the scalp when you wash your hair, so the scalp doesn't immediately go into oil production super mode. It takes some time to make the transition (it can take over a month for your scalp to really get the message), but if oil is the primary thing that's shortening the life of your blowout, the investment may be worth it.
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