Heat and Sun in Vietnam

Vietnam sits between roughly 8 and 23 degrees north of the equator. That puts the sun overhead for a meaningful chunk of the year and means UV index regularly hits 11 or 12 — the top of the scale, equivalent to skin damage in 10–15 minutes of unprotected midday exposure. Combine that with humidity in the 70–90% range for much of the year, and "it's only 32°C" turns into something that drops people from northern climates onto their backs by mid-afternoon.

This is general advice. If you have a heart condition or take medication affected by heat, talk to your doctor.

The seasons that matter

In the south (HCMC, Mekong, Phu Quoc) the toughest period is April to early June — end of dry season, no cloud cover, daily highs of 35–37°C with the heat index ("feels like") well over 40. Once the rains come in late May/June, it cools slightly but humidity peaks.

In the centre (Da Nang, Hoi An, Hue, Nha Trang) June to August is the hottest stretch, with persistent 35°C+ days and reliable afternoon sun.

In the north (Hanoi, Halong, Sapa) June to August is hottest — Hanoi regularly hits 38–40°C with stifling humidity. Hanoi summer is genuinely brutal and underestimated by people who think of Vietnam as "tropical = south."

The highlands (Da Lat, Sapa, Ha Giang) are cooler but the UV is still strong — altitude plus low latitude means easy sunburn even when it does not feel hot.

Sunscreen

Bring SPF 50 from home if you have a brand you trust. Vietnamese pharmacies and convenience stores sell sunscreen but the local market skews to "whitening" formulations and lower SPFs — finding plain SPF 50 or higher in chemist-grade chemistry is doable but not always easy. Reliable imported brands at Pharmacity and Long Chau: Anessa (Japanese), Biore UV (Japanese, very popular), La Roche-Posay Anthelios (French), Sunplay.

Apply 20 minutes before going out, reapply every two hours, and after swimming. Lips burn too — get an SPF lip balm.

A wide-brim hat beats a cap. A cap leaves your ears and neck exposed, and those are the spots that catch the worst burns on motorbikes and walking tours.

Sunglasses with UV protection prevent cumulative eye damage. Cheap ones with no UV rating are worse than nothing because they dilate the pupil without filtering the UV.

A UPF rashguard or long-sleeve linen shirt is more effective than sunscreen alone for long beach days. Most beachwear shops in Da Nang, Nha Trang, and Phu Quoc sell them.

Hydration and electrolytes

Sweating in Vietnamese humidity strips you of salt as well as water. Drinking plain water alone can actually make you feel worse — dilutional hyponatraemia is a real thing when you replace litres of sweat with plain water and nothing else.

The local solutions are easy to find:

• Pocari Sweat — Japanese isotonic, sold in any convenience store. The default electrolyte drink.
• Revive — Vietnamese isotonic, similar.
• Oresol or Hydrite — oral rehydration salts in sachet form, the most concentrated option, sold at any pharmacy for under 5,000 VND a sachet. Mix one sachet in 200–250ml water.
• Coconut water — natural and excellent, sold from carts everywhere for 25,000–40,000 VND a coconut.

A useful rule on a hot day of walking or riding: alternate between plain water and an electrolyte source. If your pee is dark yellow, drink more. If you have stopped sweating despite being hot, that is a warning sign.

Heat exhaustion — what to watch for

Heat exhaustion is the body's "I'm about to fail" signal. Symptoms:

• Heavy sweating, often suddenly profuse.
• Weakness, dizziness, light-headedness on standing.
• Headache, often pounding.
• Nausea, sometimes vomiting.
• Fast pulse, fast shallow breathing.
• Pale, clammy skin.
• Muscle cramps.

The fix: stop moving, get into shade or air-conditioning, lie down with feet slightly elevated, sip electrolyte solution slowly, and apply something cool to the neck and wrists. Most people recover within an hour. Do not push through it — recovery is dramatically slower if you do.

Heat stroke — emergency

Heat stroke is a different animal and a genuine emergency:

• Body temperature above 40°C.
• Confusion, slurred speech, agitation, or unconsciousness — this is the key marker.
• Hot, often dry skin (sweating may have stopped).
• Rapid, strong pulse.
• Possible seizures.

What to do: call an ambulance / get to A&E. While waiting, move them to the coolest place available, strip outer clothes, and cool aggressively — wet sheets, cold packs to the armpits, neck and groin, fan vigorously. Do not give fluids by mouth if they are confused. See hospitals by city for the nearest private hospital.

Heat stroke is rare in tourists but does happen — most cases I've heard of involved a midday hike on the Ha Giang loop in May, or marathon walking tours in Hanoi in July. Both are avoidable.

Cool refuges

In the hottest hours (roughly 11 am to 3 pm), most of urban Vietnam adopts a pragmatic approach: you go inside. Useful cool refuges:

• Shopping malls. Vincom, Aeon Mall, Lotte, Takashimaya — heavily air-conditioned, free to enter, fine to nurse a coffee for two hours.
• Cinemas. A 90,000–120,000 VND ticket gets you two hours of cold and a film.
• Cafes. The Vietnamese cafe culture is essentially a heat-management infrastructure. A 50,000 VND coffee buys you indefinite sitting time in air-conditioning. Highlands Coffee, Phuc Long, The Coffee House are all reliable.
• Hotel lobbies. Most are fine with non-guests sitting briefly.

Scheduling

The single biggest preventable cause of heat problems in tourists is doing the hardest physical thing in the middle of the day. Schedule:

• Walking tours, temple visits, markets — before 10 am or after 4 pm.
• Motorbike trips — early start (6–7 am), long midday break, finish before 5 pm if possible.
• Beach — fine before 10 and after 3, brutal in between. Lifeguard whistles in Mui Ne and Nha Trang are mostly directed at people not drowning but cooking.
• Treks (Ha Giang, Sapa, Cat Tien) — sunrise starts are standard for a reason.

If you get sunburnt

A burn happens to almost everyone at some point. The locally-available options:

• Aloe vera gel — every convenience store sells it. Cooling, soothing.
• Hydrocortisone 1% cream at pharmacies for inflamed areas.
• Plain paracetamol for pain.
• Stay out of the sun for several days. Cover up. Drink water.
• Watch for blistering — large blisters mean second-degree burn; do not pop, see a doctor.

For prevention reasoning and the broader health picture, see common illnesses travellers face.