Sulphur vs sulphate hot springs: smell, skin and safety

A sulphur hot spring contains dissolved hydrogen sulphide, the gas behind the rotten-egg smell and the black tarnish on silver. A sulphate spring contains sulphur that has already reacted with oxygen, so it has no smell and leaves your rings alone. Research on sulphur baths for psoriasis, eczema and arthritis is small and mostly low quality: some trials report less pain or itch, none show a cure.

What is the difference between sulphur and sulphate water?

Both contain the element sulphur, but in opposite chemical states.

Sulphur springs (also called sulphide or sulphurous springs) hold sulphur in its reduced form: hydrogen sulphide gas (H₂S) and the dissolved ions hydrosulphide (HS⁻) and sulphide (S²⁻). Which form dominates depends on pH. In acidic water it stays almost entirely as H₂S gas; the more alkaline the water, the more of it sits as dissolved HS⁻ [1]. For bathers the gas form matters most, because H₂S can pass through the skin into the dermis rather than staying on the surface [1].

Sulphate springs hold sulphur as the sulphate ion (SO₄²⁻), which is where hydrogen sulphide ends up once it oxidises [1]. It is the same ion found in Epsom salt (magnesium sulphate), and it has no smell.

Japan has detailed official rules for this, and it defines the two very differently [2]:

• A sulphur spring (硫黄泉, iō-sen) needs at least 2 mg of total sulphur per kg of water, counting only hydrogen sulphide, hydrosulphide and thiosulphate. Sulphate does not count. If free H₂S gas is the main form, the label adds “hydrogen sulphide type”.
• A sulphate spring (硫酸塩泉) needs at least 1,000 mg/kg of dissolved minerals, with sulphate as the main negative ion. It is then named after its main positive ion: sodium, magnesium, calcium, iron or aluminium sulphate spring.

So a sulphur spring can be quite dilute and still qualify, while a sulphate spring is mineral-heavy by definition.

Why do some hot springs smell like rotten eggs?

The smell is hydrogen sulphide. Some people notice it at 0.01 ppm, far below harmful levels [3], which is why even a weak sulphur spring announces itself from the car park. Sulphate has no smell, so a sulphate spring without hydrogen sulphide smells of little more than warm water.

The smell is not a reliable gauge of danger. At 100 to 150 ppm hydrogen sulphide knocks out your sense of smell [3], and with longer exposure you stop noticing even low levels [4]. More on that in the safety section.

Will a sulphur spring tarnish my silver jewellery?

Very likely. Hydrogen sulphide, together with the related gas carbonyl sulphide, is one of the main causes of silver corrosion in air, working in moderate to high humidity. The dark film is mostly silver sulphide [5]. Warm, steamy air over a sulphur pool is close to ideal for it. Take off silver rings, chains and anklets before you go in and keep them sealed in a bag, not on a rock by the pool.

Sulphate is not the culprit: silver sulphate is essentially absent from tarnish layers [5]. If your silver darkens at a spring sold as “sulphate”, there is hydrogen sulphide in the water or the air too.

Sulphur vs sulphate springs at a glance

	Sulphur (sulphide) spring	Sulphate spring
Form of sulphur	Hydrogen sulphide gas, hydrosulphide, sulphide	Sulphate ion (SO₄²⁻)
Smell	Rotten eggs, even when weak	None
Silver jewellery	Turns black	Not affected by the sulphate itself
Japanese threshold	2 mg/kg total sulphur in sulphide forms [2]	1,000 mg/kg dissolved minerals, sulphate dominant [2]
Japan’s official bathing uses	Atopic dermatitis, psoriasis, chronic eczema, skin infections [6]	Same as salt springs: cuts, poor circulation, cold hands and feet, low mood, dry skin [6]
Japan’s bathing cautions	Sensitive skin or mucous membranes; dry skin in older people [6]	No type-specific caution
Best clinical evidence	Small, mostly low-quality trials in skin and joint disease	Mostly from drinking, not bathing

Japan’s list of uses is a set of official posting rules for bathhouses, not proof that the water works. The same notice says the effect of a spa stay comes from the water, the heat, the setting and the change of routine together, and that it is hard to pin any benefit on the water’s chemistry alone [6]. The next two sections cover what trials have actually found.

Do sulphur springs help psoriasis, eczema or acne?

The idea has a plausible basis. Sulphur loosens the outer layer of skin (it is keratolytic) and has antibacterial and antifungal activity, and sulphur products have long been used for acne, rosacea, seborrhoeic dermatitis and dandruff [7]. But that evidence comes from creams and washes made for the purpose, not from soaking in spring water with a few milligrams of sulphur per litre.

For bathing, the evidence is thin:

• Psoriasis. A 2024 systematic review found eight trials of thermal mineral water baths for skin disease: seven in psoriasis, several of which paired the baths with UV light therapy, and one in eczema. All reported improvement, but the authors said the studies were too limited and often too low in quality to count as robust evidence, and they did not pool the results [8]. The one included trial named for sulphur water, 35 adults with plaque psoriasis in Italy, had no control group and was rated at high risk of bias [8].
• Eczema (atopic dermatitis). In a study at a university hospital branch in Kusatsu, Japan, 70 people with hard-to-treat atopic dermatitis bathed in acidic hot-spring water at 42°C for 10 minutes, twice a day. Skin improved in 76%. Of the 42 who improved and were tested, 30 had less or no Staphylococcus aureus on their skin afterwards [9]. There was no control group, and the water was acidic, which most Malaysian springs are not.
• Acne. We found no clinical trials of bathing in sulphur springs for acne. The evidence for sulphur in acne comes from topical products [7].

The same chemistry can irritate. Japan lists sensitive skin as a reason to avoid sulphur springs [6], and doctors have described a 65-year-old man who developed punched-out ulcers on his thighs and shins after bathing in a hot sulphur spring, with tissue findings matching a superficial second-degree burn [10]. It is a single case, but it shows that hot sulphur water can damage skin.

Are sulphur baths good for arthritis and joint pain?

Joints are where spa therapy has been studied most, and the results are mixed:

• A 2007 Cochrane review of balneotherapy for osteoarthritis (7 trials, 498 patients) found no significant difference in pain or function at one or three months between sulphur baths and no treatment. It found some benefit for mineral baths in general over no treatment, but rated the overall evidence weak because of poor study quality [11]. Its searches end in October 2006.
• In a double-blind trial that enrolled 47 people with hand osteoarthritis, those who bathed in sulphurous thermal water 15 times over three weeks had less pain at three months than those bathing in warm tap water [12].
• In rheumatoid arthritis, a 1990 Dead Sea study split 40 patients into four groups of 10: mud packs, hot sulphur baths, both, or neither. The three treated groups improved on most clinical measures for up to three months; blood markers did not change [13]. A 2015 Cochrane review of 9 trials (579 people) in rheumatoid arthritis concluded the evidence is not enough to show that spa baths work better than no treatment [14].
• A 2017 meta-analysis of 8 knee osteoarthritis trials (734 people) found balneotherapy improved pain, stiffness and function scores, but results varied so much between studies that the authors would only say it “may” help [15]. It covered spa bathing in general, not sulphur water in particular.

The fair reading: warm mineral baths may ease joint pain for weeks to a few months. Whether the sulphur adds anything beyond warmth, buoyancy and rest is not settled. Work on hydrogen sulphide and inflammation is mostly in cells and animals, and even a review that favours sulphur waters describes the outlook as “promising”, not proven [1].

What are sulphate springs good for?

In Japan, sulphate springs get the same bathing uses as salt (chloride) springs: cuts, poor circulation, cold hands and feet, low mood and dry skin. Their extra listed uses are for drinking: constipation, high cholesterol and gallbladder problems [6].

The constipation part has some trial support. A French randomised trial enrolled 244 women with functional constipation. By week two, 37.5% of those drinking 1 litre a day of a magnesium sulphate-rich mineral water had less constipation, against 21.1% of those drinking low-mineral water [16]. One of the authors worked for the company that sells the water, and it was bottled water made for drinking. The water is rich in both magnesium and sulphate, so the trial can’t say which did the work; our magnesium guide covers what bathing in it can and can’t do.

Don’t drink from Malaysian hot springs. The authors of a 2013 study of Selangor springs advise against it because the water did not meet Malaysia’s national standard for drinking [17].

Which Malaysian hot springs are sulphurous?

Malaysia’s hot springs, outside Sabah, are not volcanic: the water is heated deep in the crust, and the springs come out near neutral rather than acidic [18]. In Selangor most sit along the granite of the Main Range [17]. The most detailed local sulphur data is a 2013 UiTM study of springs in Selangor and Kuala Lumpur [17]. Total sulphur (all forms) ranged from 1.87 to 12.41 mg/L in filtered samples, while sulphate was only 0.15 to 1.51 mg/L. For comparison, the same paper lists Kusatsu in Japan at 611 mg/L of sulphate.

Total sulphur by spring, from that study:

• Selayang: about 11 to 12 mg/L
• Kerling: about 11.6 mg/L
• Sungai Serai, Hulu Langat: about 11.5 mg/L
• Hulu Yam: about 10.4 mg/L
• Kuala Kubu Bharu: about 10 mg/L
• Setapak: 5.3 to 10.5 mg/L across two pools
• Hulu Tamu: 1.9 to 11.9 mg/L across two pools

None of these is a sulphate spring in the Japanese sense: dissolved solids were 225 to 376 mg/L, far below the 1,000 mg/kg bar, and sulphate was a trace [2] [17]. That leaves most of the sulphur in other forms. The study did not measure hydrogen sulphide on its own, so it cannot tell you whether each spring would pass Japan’s 2 mg/kg sulphur test.

Treat the figures as a hint, not a label, because they shift with the lab method. A 2017 study measured total sulphur at only 0.5 mg/L at Sungai Serai, against about 11.5 mg/L in 2013 [18]. The 2017 study also found sulphate at 1 to 8 mg/L at Ulu Slim, Sungai Klah (Felda Residence), Dusun Tua and Sungai Serai [18], and a 2022 study at Lojing in Kelantan averaged 18 ppm [19]. The odd one out in 2017 was the saline spring at Ayer Hangat, Langkawi, with 947 mg/L of sulphate. Chloride (about 13,800 mg/L) outweighs it many times over, so by our reading of Japan’s rules it would be a salt spring that contains sulphate, not a sulphate spring [18].

Sabah is different. Tawau Hills Park sits on volcanic rock, and both of its spring areas are classed as acid sulphate water [20]:

• The sulphur springs on the main trail, along the upper Tawau River, are only warm (about 31°C at the one spring measured) and acidic, with a pH around 4. You can smell hydrogen sulphide 150 m before the first spring, and the boulders around them are coated in sulphur [20].
• The Balung springs, a four-hour trip from park headquarters, are 50 to 56°C, close to neutral and also smell strongly of hydrogen sulphide. They carry about 1,250 to 1,300 mg/L of sulphate and about 1,840 mg/L of total solids, mostly calcium sulphate [20]. By our reading of Japan’s rules, that makes them a calcium sulphate spring, the only Malaysian example we found in published data.

Elsewhere, the sulphur label rests on operators, visitors or field notes rather than water analysis. ēRYA by Suria Hot Spring Bentong describes its water as sulphuric, Beacon Resort lists sulphur among its minerals, Gadek is reported to smell of hydrogen sulphide, Jeli is described as high in sulphur, and a sulphur smell is reported at La. At Sungai Mering, a 2002 geological survey noted a strong sulphur smell and sulphur deposits. Poring in Kinabalu Park pipes its sulphurous water into tubs and enclosed private bath rooms. If the label matters to you, your nose and your silver are better guides than the signboard.

Is hydrogen sulphide at hot springs dangerous?

In open-air pools it is usually just a smell. But the gas is toxic at higher concentrations, and the warning signs are easy to miss:

• Long exposure at just 2 to 5 ppm can cause headaches, nausea and watery eyes, and can tighten the airways of some people with asthma. Dizziness is listed from 20 ppm, eye and throat irritation from 50 ppm, and above 500 ppm people can collapse within minutes [3].
• It is heavier than air and builds up in low-lying areas and confined spaces [4].
• Your nose stops working as a detector, quickly at high levels and gradually at low ones [4].

Japan regulates this. For any spring with 2 mg/kg or more of total sulphur, bathhouses must keep hydrogen sulphide below 20 ppm at 10 cm above the water and below 10 ppm at 70 cm above the floor. Bathrooms that rely on vents need at least two, one of them at floor level [21]. People have died: the environment ministry’s records list four deaths from hydrogen sulphide in 2005 in a car park near an inn at Doroyu hot spring [22].

At Malaysian sulphur springs:

• Prefer open-air pools. Take more care in small enclosed bath rooms, especially if the smell is strong and there is no fan or window.
• Don’t lean down over a bubbling spring eye or into a pit or hollow beside it.
• Get out if you feel a headache, dizziness, nausea or stinging eyes, or if a strong smell seems to fade while you have not moved.
• Keep children back from vents and spring eyes.

Who should be careful with sulphur springs

Japan’s posting rules list these reasons to avoid any hot spring: active illness (especially with fever), active tuberculosis, advanced cancer or severe anaemia, heart or lung disease bad enough that light activity leaves you breathless, severe kidney disease with swelling, bleeding from the gut or any visible bleeding, and a flare-up of a chronic illness [6]. For sulphur springs they add [6]:

• People with sensitive skin or mucous membranes.
• Older people with dry skin.

The same rules advise people with delicate skin to rinse off with warm water after strong waters such as sulphur or acidic springs, and to drink a glass of water after bathing [6]. In Malaysian heat, the water matters even more.

People with asthma should also be wary of strongly smelling springs, since even low levels of hydrogen sulphide can narrow the airways in some asthma patients [3].

Talk to your doctor first if your psoriasis or eczema is cracked, weeping or infected, or if you have heart or lung disease. A soak can sit alongside treatment, but it does not replace it. Our hot spring safety guide covers heat, infections and fainting.