Green patch under lifted gel: usually bacteria, not fungus

If you take off your gel and find a green patch under the nail, it is usually not a fungus or “black mould”, whatever the internet says. It is most often a bacterium, Pseudomonas aeruginosa, which makes a blue-green pigment of its own. Doctors call it green nail syndrome, or chloronychia. The right move is to stop gel on that nail, not to paint over it, and to see a doctor. This lesson explains what causes it, why it likes lifted gel, and what to notice.

It does not teach diagnosis. Nails change colour for many reasons, some of them serious, and only a doctor can say which one you have.

Where the green comes from

P. aeruginosa is an opportunistic bacterium. It does well in damp places and grows best at about 37 °C, right at body temperature. It produces two pigments, pyoverdine and pyocyanin, which dissolve in water and stick to the underside of the nail plate. That is why the colour looks like it is “inside” the nail and will not file or wipe off. One case report describes it starting bright green and turning duller over time.

A 2025 case report in Cureus gives a sense of scale: the condition accounts for about 1-4% of all nail infections, mostly in adults aged 40 to 60, with only 5-8% of cases in people under 30. People whose hands are wet a lot, such as dishwashers, bakers, barbers and healthcare staff, get it more often than the general population.

Why it turns up under lifted gel

A report in the journal Dermatology Review notes that this infection rarely involves intact nails. It needs a gap. When the nail separates from its bed (onycholysis), or gel lifts off the nail plate, the space underneath collects water, dirt and skin debris, and stays warm and damp. The same report cites earlier work finding onycholysis in about 77% of people with this infection.

On artificial nails and gel, the evidence points both ways:

• Patient Care notes that people with artificial nails appear predisposed to this infection, possibly from small injuries during application and extra moisture. It describes a 29-year-old woman whose nail turned green 6 weeks after artificial nails were applied.
• A 2026 report in Oxford Medical Case Reports describes a healthy 17-year-old who developed it after a single gel manicure. The authors think the procedure caused microtrauma that let the germ in, and stress proper sterilisation of nail tools.
• On the other side, American Family Physician says artificial nails and polish “may” increase the risk, but there is little evidence to confirm it.

The fair summary: gel itself does not infect a nail. Lifted gel left in place, picking, and dirty tools create the gaps and small wounds this germ likes.

Worked example: how far has your nail grown?

A 2010 study of 22 healthy young adults found fingernails grow an average of 3.47 mm a month. Here is how wide the bare strip at the base gets over time (counting 30 days to a month).

Gel worn for	Calculation	New nail at the base, roughly
2 weeks (14 days)	3.47 × 14 ÷ 30	1.6 mm
3 weeks (21 days)	3.47 × 21 ÷ 30	2.4 mm
4 weeks (28 days)	3.47 × 28 ÷ 30	3.2 mm
6 weeks (42 days)	3.47 × 42 ÷ 30	4.9 mm

These are averages from a small sample, but they make a useful reminder. A set with an almost 5 mm gap at the base has been on about as long as the 6-week case above. Whenever you see an edge lifting, do not wait for your next appointment: remove the gel properly and look at the real nail underneath.

Which green, and what to tell the doctor

This table is not a diagnostic tool. It is a guide to what to describe, and what not to wait on. It draws on American Family Physician and Patient Care.

What you see	What often comes with it	What to do
Green or blue-green under the lifted part	Skin around the nail looks normal, usually no pain or fever	Stop gel on that nail, book a doctor
Green with swelling, redness, heat, pain or pus	Inflammation around the nail	See a doctor the same day
Thick, cloudy, crumbly nail	Could be a fungal infection, treated differently	Have a doctor check. Do not buy cream and self-treat
Colour that comes off with polish remover	Usually dye from polish or chemicals	Remove all colour and look again
Red-purple or black after a knock	Could be blood under the nail	If unsure, have a doctor look
A dark streak or patch with no cause	Can be a sign of melanoma of the nail	See a dermatologist soon

If your immune system is weakened, for example by immune-suppressing drugs or chemotherapy, tell your doctor promptly, because this germ takes advantage of exactly that. For people with normal immunity, American Family Physician says the risk of it spreading into the body is low.

For the other nail signs worth a visit, see Nail changes that need a doctor.

What to do if you find one

1. Stop gel on that nail. Do not paint a dark colour over it. The doctor needs to see the real nail.
2. Do not pick off lifted gel. Picking tears off the top of the nail and widens the gap. For your other nails, follow Removing gel safely. For the green one, ask the doctor how it should come off.
3. Keep it dry. Dry your hands after washing, wear gloves for dishes, avoid knocking that finger.
4. Take a photo and note the dates: when the gel went on, when you noticed lifting, when you saw the green. This helps the doctor a lot.
5. See a doctor. Treatment depends on severity. A doctor may trim away the separated part and prescribe a cream or tablets. In the case reports above, the colour improved within about 10 days to 4 weeks of the treatment the doctor prescribed, but everyone’s timeline differs.

A green nail is not a reason to panic, but it is not something to paint over and forget either. Stop gel on that nail and let a doctor look.

Common mistakes

• Assuming it is fungus and self-treating with antifungal cream. The usual cause is a bacterium. Antifungal cream is not made for it, and self-treating wastes time before a proper check.
• Painting a dark colour over it. That keeps the moisture locked in and hides what the doctor needs to see.
• Home bleach or vinegar soaks. Their effectiveness has not been reported, and they can irritate the skin around the nail.
• Leaving lifted gel on until the next appointment. The gap under a lifted edge holds water. The longer it stays, the higher the risk.
• Using one file on every nail without cleaning it. See Cleaning and disinfecting nail tools.

Keeping it from coming back

Cap the free edge every time you apply gel. Do not pick or prise up lifting gel. Dry your hands properly, including around the nails. Wear gloves for wet work. Keep nails short while a nail that has separated grows out, and clean your tools every time. If a nail has lifted off its bed, let it grow out and reattach before going back to gel, and check with the doctor who treated you first.

Sources

• Green nail syndrome — American Family Physician (AAFP)
• Green nail syndrome — Patient Care
• Chloronychia: Green Nail Syndrome Caused by Pseudomonas aeruginosa in a Younger Patient — Cureus (PMC)
• Onycholysis associated with green nail dyschromia — Dermatology Review
• Green nail syndrome following nail art — Oxford Medical Case Reports (PMC)
• Growth rate of human fingernails and toenails (Yaemsiri et al., 2010) — US EPA HERO