The Complete Underwear Guide: Gusset Fabric, Washing, Replacement and Period Underwear
You wear it against your skin for more than ten hours a day, yet underwear may be the one thing on your body that almost nobody looks into. We read ingredient lists before buying skincare and compare formulas before choosing a shampoo. When it comes to underwear, the decision usually comes down to three things: price, style and whether it looks nice.
This guide won't tell you to buy one particular kind of underwear. Instead, it lays out the trade-offs behind each choice, because what suits you depends on your body, your daily routine and Hong Kong's climate.
This article draws on guidance from the American College of Obstetricians and Gynecologists (ACOG), published laboratory studies and other public sources, listed at the end. It was compiled by SERENE. The author is not a healthcare professional, and the content is not a substitute for a doctor's diagnosis.
Key points:
- The gusset matters most. Lace or mesh on the outside is fine; choose cotton for the gusset that sits against your skin.
- A good fit is not a tight fit. Friction and trapped heat from tight underwear are behind a lot of everyday discomfort.
- Machine or hand washing both work. What matters is rinsing thoroughly, skipping fabric softener and drying completely.
- After treatment for a yeast infection, replace your underwear. Home laundry temperatures may not remove Candida from fabric.
- Period underwear: check whether PFAS have been intentionally added before you buy.
- Paper underwear is for emergencies only. For travel, disposable cotton underwear is the more practical choice.
Contents
- The gusset: the most important part of your underwear
- Fabrics compared
- Styles: are thongs really bad for you?
- Sizing: the underrated cost of going too tight
- Machine wash or hand wash?
- When to replace your underwear (including after an infection)
- Period underwear: worth it, but ask one question first
- Disposable underwear: paper and cotton are not the same
- What to wear in seven situations
- Hong Kong's climate: the variable nobody talks about
- Five myths
- Shopping checklist
Chapter 1 | The gusset: the most important part of your underwear
If you remember only one thing from this guide, make it this.
The gusset is the panel in the middle of your underwear that sits directly against the vulva. Whether the outer fabric is lace, silk or mesh, it is the material and breathability of the gusset that really decide how comfortable you feel all day.
Why this part?
- It is the only part in prolonged, direct contact with the skin around the mucous membranes
- Vulvar skin is thinner than skin elsewhere on the body and reacts more to friction and damp heat
- Discharge, sweat and menstrual blood collect here first
This also clears up a common misconception: lace underwear is not the problem in itself. The outer fabric matters far less than the gusset. What is worth avoiding is underwear made entirely, gusset included, of non-breathable synthetic fibres. ACOG also recommends breathable cotton underwear, mainly to reduce heat and irritation[3]. To understand the basic difference between the vulva and the vagina, see our intimate care guide for women in Hong Kong (in Chinese).
In one line: wear whatever you like on the outside, but choose cotton for the gusset.
Chapter 2 | Fabrics compared
| Fabric | Breathability | Absorbency | Drying speed | For the gusset |
|---|---|---|---|---|
| Cotton | High | High | Slow | ✅ Best all-round |
| Organic cotton | High | High | Slow | ✅ Same as cotton |
| Modal / Tencel | High | High | Medium | ✅ Softer |
| Bamboo fibre | Medium–high | High | Medium | ⭕️ Acceptable |
| Silk | Medium | Medium | Medium | ⭕️ Hard to care for |
| Nylon / polyester | Low | Low | Fast | ❌ Not recommended |
| Elastane (spandex) | Low | Low | Fast | ❌ Blends only |
Two things worth clarifying
"Bamboo" usually isn't what you think. Most bamboo underwear is actually a regenerated fibre (viscose) made from bamboo through a chemical process. It does feel soft and absorbs well, but claims of being "naturally antibacterial" are hard to sustain once the fibre has been processed.
Be sceptical of "antibacterial underwear". Underwear treated with silver ions or other antimicrobial agents may reduce bacterial attachment in the lab, but the evidence that this makes any real difference to everyday health is limited. More importantly, your intimate area needs its normal microbial community. Underwear that keeps killing bacteria may not be what you actually want.
Chapter 3 | Styles: are thongs really bad for you?
This is the most widely misunderstood question. The idea that thongs carry bacteria from back to front and cause infections is everywhere, but research results are inconsistent. Several studies looking at thongs and urinary tract infections or bacterial vaginosis have not found a clear link.
What matters is not the style but three conditions:
- Is it too tight? Friction and pressure are what cause irritation
- What is the gusset made of? Thongs can have cotton gussets too
- How long do you wear it? All day is different from occasionally
| Style | Pros | Watch out for |
|---|---|---|
| Briefs | Balanced, good for everyday | Replace once the waistband loosens |
| Boyshorts | Least friction | Legs may roll up |
| High-waisted | No pressure on the abdomen | A tight waistband can dig in |
| Thong | No visible lines under fitted clothes | Choose a cotton gusset, avoid tight fits, not ideal for exercise |
| Seamless (bonded) | Invisible under clothes | Usually synthetic; not for long wear |
Chapter 4 | Sizing: the underrated cost of going too tight
People often size down not to look slimmer, but because "it'll stretch anyway". Underwear that is too tight causes three real problems:
Constant friction The skin barrier of the vulva wears down over time, making redness and stinging more likely.
Trapped heat Pressure plus poor airflow creates a warm, damp environment.
Marks at the groin Long-term pressure can lead to inflamed hair follicles or darkened skin. Freshly waxed or shaved areas are especially sensitive; see our guide to aftercare after intimate hair removal (in Chinese).
Three self-checks:
- When you sit down, does the waistband dig into your skin?
- After taking them off, are there marks that last more than ten minutes?
- Do you keep adjusting them while you walk?
If any of these apply, the size is wrong. It is not a problem with your body.
Chapter 5 | Machine wash or hand wash?
This is the question people ask most, and opinions vary. The short answer: both work. What matters is not your hands versus the machine, but three things: rinsing thoroughly, skipping fabric softener and drying completely.
Machine washing
Pros Plenty of water flow and a thorough rinse, which is actually hard to achieve by hand. It saves time, so you're more willing to wash often.
Cons Waistbands and lace trims can stretch out of shape; washing with other items carries some risk of cross-contamination.
How to machine wash properly
- Always use a mesh laundry bag, the cheapest way to make underwear last longer
- Wash separately from socks, cleaning cloths and floor mats; this matters far more than water temperature
- Use around 30°C on a gentle or delicates cycle
- Don't overload the machine; a full drum doesn't rinse properly
- Add an extra rinse if your machine has the option
Hand washing
Pros Gentlest on waistbands and lace, noticeably extending their life; lets you treat individual stains.
Cons The biggest risk is incomplete rinsing. Detergent left in fabric that sits against your skin is more likely to cause irritation than the detergent itself, and this is the step most people skip when washing by hand.
How to hand wash properly
- Use warm water, not hot, around 30°C. Hot water ages elastic fibres faster
- Use a small amount of detergent; more is harder to rinse out, not cleaner
- Soak for no more than 15 to 30 minutes; longer soaking adds nothing
- Rinse at least three times, until there are no suds at all; take this step seriously
- Don't wring hard; roll them in a towel to absorb water and protect the elastic
Period stains: always use cold water
Hot water makes the proteins in blood set, fixing the stain into the fibres and making it harder to remove. Rinse with cold water as soon as you can, rub gently, then wash as normal. This applies to both machine and hand washing.
Three habits that don't help, and may make things worse
❌ Using bleach
It damages fibres and elasticity, and residue can irritate skin. Light patches on the gusset of dark underwear come from the mildly acidic nature of vaginal discharge fading the dye. That is normal and doesn't need to be "washed out".
❌ Using fabric softener
Softener leaves a film on fibres that reduces absorbency, cancelling out the reason you chose cotton in the first place. Fragrance residue is also a common irritant.
❌ Believing "the hotter, the cleaner"
High temperatures do reduce microbes, but they also age elastic fibres faster. For everyday washing, rinsing and drying well matter more than heat. (After an infection, things are different; see Chapter 6.)
Drying: where things most often go wrong in Hong Kong
- Put them away only when completely dry; half-dry fabric is exactly what mould likes
- Dry in a ventilated spot or in sunlight; UV light and dryness are the most practical treatment
- Avoid high-heat tumble drying; use low heat or air dry
- Hang underwear in the airflow of a running dehumidifier, a practical way to dry indoors in Hong Kong
Chapter 6 | When to replace your underwear
At normal wearing frequency, it's generally suggested that you review your underwear every 3 to 6 months. But the calendar is only a reminder to check. The real test is these four signs, and if any one applies, it's time to retire the pair:
- The waistband has lost its stretch or become wavy
- The gusset is discoloured, stiff or has marks that won't wash out
- The fabric is pilling, or has become noticeably thin or see-through
- You keep having to pull and adjust them
How often you wear a pair changes this a lot. A pair worn twice a week will naturally last longer than one worn five times a week. So go by the signs, not just the calendar.
A practical tip
Buy six to eight pairs of the same style at once and write the date on the label. When the time comes, check them all together instead of judging each pair separately. That way you won't keep putting it off because they're "still wearable".
A special case: after an infection, start with a new set
If you've had a yeast (Candida) infection, we recommend replacing your underwear with a new set once treatment is finished, rather than continuing to wear the old ones.
The reason: home laundry temperatures may not be enough to remove Candida left on fabric. Laboratory studies have found that contaminated fabric washed at 40°C still carried living fungi, and that reliable decontamination required 60°C[1][4]. Most underwear care labels don't allow washing at 60°C.
Replacing them is the simplest and most thorough option: no need to check each care label, and no worrying about which pair wasn't treated properly. For other infections such as bacterial vaginosis, there is even less evidence that clothing plays a role, but replacing your underwear costs little and is still worth considering.
Three tips when replacing
- During treatment, change daily as usual, wash used pairs separately from other clothes, and retire them all once treatment ends
- Choose cotton gussets for the new set and wash them once before wearing
- Keep towels to yourself too, wash them often and dry them completely
To be clear: replacing your underwear supports treatment but cannot replace it. Most recurrences are linked to the body itself rather than clothing. To understand why yeast infections come back, read why Candida keeps returning (in Chinese). If discomfort keeps recurring, see When to see a doctor below.
Chapter 7 | Period underwear: worth it, but ask one question first
Period underwear uses several layers of fabric to absorb menstrual blood and can be washed and reused. It doesn't have to replace pads; more often, the two are used together. For a comparison of period underwear with pads, tampons and menstrual cups, see our period products comparison (in Chinese).
Good for Lighter days at the start and end of your period; backup against leaks alongside a menstrual cup or tampon; less worry about leaks overnight.
Watch out for On heavy days it may not be enough on its own. Follow the brand's guidance on how long to wear it, and change more often the heavier your flow.
PFAS: ask before you buy
PFAS are a group of water- and stain-repellent chemicals, known as "forever chemicals" because they barely break down. A 2025 study from the University of Notre Dame tested 59 reusable period and incontinence products and found that about one-third of the period underwear had fluorine levels high enough to indicate PFAS had been added intentionally[2].
The same study also found that most products showed no sign of intentional addition, which means PFAS-free options do exist. The risk of absorbing PFAS through the skin is still not well understood, but when alternatives are available, avoiding them is a reasonable choice.
Three things to check before buying
- Does the brand clearly state "no intentionally added PFAS", and can it provide test reports?
- Is the layer against your skin cotton?
- If it mentions a "stain-resistant coating" or "waterproof treatment", ask what that treatment is
How to wash
- Rinse in cold water until it runs clear, then machine or hand wash according to the label
- No fabric softener; it clogs the absorbent layer and reduces absorbency
- No hot water or high-heat drying, to avoid damaging the leak-proof layer
- Put away only when fully dry; the thick absorbent layer dries especially slowly in Hong Kong's humidity
- Replace when absorbency drops noticeably or a smell won't wash out
Chapter 8 | Disposable underwear: paper and cotton are not the same
"Disposable underwear" actually covers two very different things, used in different ways.
| Paper underwear | Disposable cotton underwear | |
|---|---|---|
| Material | Non-woven, mostly synthetic | Mainly cotton |
| Breathability and absorbency | Low | Close to regular cotton underwear |
| Durability | Tears and creases easily, more friction | Lasts a full day |
| Best for | Short-term emergencies, hospital stays | Travel, business trips, changing after exercise |
Paper underwear solves convenience, not comfort. It's fine for the occasional emergency, but not recommended for wearing all day, especially in hot, humid weather. For travel, disposable cotton underwear is the more practical choice, and it also solves the problem of underwear not drying in your hotel room. For care during a hospital stay after giving birth, see our postpartum intimate recovery guide (in Chinese).
When choosing disposable cotton underwear
- Check that the label shows mainly cotton, and that the gusset is cotton
- Look for individually wrapped pairs with the sterilisation method stated. Disposable doesn't mean sterile; wearing them straight from the pack depends on reliable packaging and processing
- Avoid scented products
Chapter 9 | What to wear in seven situations
| Situation | Suggestion |
|---|---|
| Work days | Cotton gusset, a good fit without tightness. Keep a spare pair at the office |
| Exercise | Quick-dry fabrics are fine, but change straight after. Damp, heat and friction together are the worst combination |
| Your period | Loose, dark-coloured, breathable gusset. The vulva is more sensitive now, so avoid rubbing lace trims. For period underwear, see Chapter 7 |
| Sleep | You can go without. Eight hours of dryness isn't indulgence; it lets the skin barrier rest |
| Travel | Prioritise quick-dry, but keep a cotton gusset; disposable cotton underwear is also a good option (see Chapter 8). On long-haul flights, change once midway |
| Swimming | Change into dry underwear as soon as you're out of the water. A wet swimsuit worn for hours is one of the most common triggers |
| Humid spring days | Make sure underwear is completely dry before wearing. Half-dry is worse than not changing |
Chapter 10 | Hong Kong's climate: the variable nobody talks about
Relative humidity in Hong Kong often goes above 90% in summer. That means three things:
① Longer drying times
Underwear dried indoors may take two days to dry properly. Putting it in a drawer half-dry simply locks the damp in.
② Repeated sweating from temperature swings
From the street into an air-conditioned MTR carriage and back out again: you sweat, stop, then sweat again throughout the day, and the gusset collects far more moisture than you'd think.
③ Walls and wardrobes turn damp in humid spring weather
Clothes stored at this time of year can pick up a musty smell, so make extra sure your underwear is dry.
Three Hong Kong-specific habits
- Hang underwear in the airflow of a running dehumidifier
- Keep moisture absorbers in your wardrobe and replace them every season
- Carry a spare pair in summer. It's the cheapest, most direct thing you can do
For more on intimate care in hot, humid weather, read our guide to summer itching and discomfort (in Chinese).
Chapter 11 | Five myths
Myth 1 | The tighter the fit, the better
Friction and trapped heat from tight underwear are behind a lot of everyday discomfort. A good fit and a tight fit are two different things.
Myth 2 | Daily panty liners keep you cleaner
Wearing a liner every day adds a non-breathable layer and extra friction every day. If you use them because of heavier discharge, it's worth finding out why rather than covering it up. Start with our article on whether heavy discharge is normal (in Chinese).
Myth 3 | Light patches on dark underwear mean it isn't clean
The mildly acidic nature of discharge fades the dye. It's normal and has nothing to do with how well you wash.
Myth 4 | Sleeping without underwear is unhygienic
The opposite: giving the vulva some time to stay dry benefits the skin barrier, as long as your bedding is kept clean.
Myth 5 | Lace underwear is always bad for you
The deciding factors are the gusset material and the fit, not the pattern on the outside. A lace pair with a cotton gusset is much closer to plain cotton underwear than you might think.
Chapter 12 | Shopping checklist
Next time you buy underwear, check these six things in order:
- Is the gusset cotton? Turn it inside out and look, rather than relying on the product name
- Is the fabric content clearly labelled? Vague terms like "comfort fabric" usually mean synthetics
- Is the elastic covered or exposed? Covered elastic causes less friction
- Are the seams on the inside? Exposed seams against the skin cause discomfort over time
- Is the size right? Judge it sitting down, not standing up
- Does it smell scented? Fragrance on new underwear signals finishing chemicals, so always wash before wearing
Cotton gusset, the right size, rinsed thoroughly, completely dry, replaced when the signs appear.
Five things, and none of them is difficult.
When to see a doctor
The right underwear and better laundry habits can deal with everyday factors like friction and heat. The situations below are not something new underwear can fix. To tell common causes apart, see our guide to itching, odour, discharge and painful urination (in Chinese).
You don't need to wait until things feel "serious". Family doctors, gynaecologists and the Family Planning Association of Hong Kong's clinics can all help. For changes in odour, you can start with our guide to telling normal from abnormal odour (in Chinese).
Before your appointment, note down
- When the discomfort started, and whether there's a pattern (for example, around your period)
- Whether you've used any medication or cleansing products yourself
- Whether you've recently switched underwear, laundry detergent or hygiene products
For the moments that are hard to talk about, you don't have to guess alone.
SERENE is a Hong Kong intimate care brand. We believe caring for your intimate area should feel as ordinary as caring for the skin on your face. That's why we keep writing about the questions that are often skipped over but affect you every day, clearly and with evidence.
Follow @serene.asia on Instagram →FAQ
How often should I replace my underwear?
Review it every three to six months, then decide based on signs such as a stretched waistband, a discoloured or stiff gusset, or thinning fabric. See Chapter 6.
Can I keep wearing my underwear after a yeast infection?
We recommend replacing it with a new set once treatment is finished. Home laundry temperatures may not remove Candida from fabric, and replacing is the simplest, most thorough option. See Chapter 6.
Can I wash underwear with other clothes?
Yes, with regular clothing, but put it in a mesh bag and keep it away from socks, cleaning cloths and floor mats.
How many times a day should I change my underwear?
Once a day is usually enough. Change straight away after exercise or swimming, or when you've been sweating a lot or have more discharge.
Should I wash new underwear before wearing it?
Yes. New underwear may carry finishing chemicals from manufacturing, so wash it once first.
Can I wear period underwear all day?
It depends on your flow and the brand's guidance. On heavy days, change more often or use it with other period products. When buying, check that no PFAS have been intentionally added.
Can I wear paper underwear every day?
It's not recommended. Paper underwear is low in breathability and absorbency, and is best for short-term emergencies. For travel, use disposable cotton underwear instead.
Do thongs cause infections?
Current research hasn't found a clear link. What matters is the fit, the gusset material and how long you wear them, not the style itself.
Related reading (in Chinese)
References
- Ossowski B, Duchmann U, Boslet W. Disinfecting treatment of textiles to prevent reinfection in vulvovaginal candidiasis [Desinfizierende Behandlung von Textilien zur Rezidivprophylaxe bei vulvovaginalen Candidosen]. Geburtshilfe und Frauenheilkunde. 1999;59(4):175–179. doi:10.1055/s-1999-14184.
- Wicks A, Venier M, Peaslee GF, et al. PFAS in reusable menstrual and incontinence products. Environmental Science & Technology Letters. 2025. See also: American Chemical Society. Research Update: PFAS found in several reusable period products. August 2025.
- American College of Obstetricians and Gynecologists (ACOG). Vaginitis. Patient FAQ.
- Rashid S, Collins M, Corner J, Morton RS. Survival of Candida albicans on fabric after laundering. British Journal of Venereal Diseases. 1984;60(4):277. doi:10.1136/sti.60.4.277.
This article is for general health education only. It does not constitute medical advice, diagnosis or treatment, and is not a substitute for advice from a registered doctor. If you have any discomfort or concerns, please consult a gynaecologist or another qualified healthcare professional.