Did the brand add limonene deliberately?

Usually not as such. It normally comes in as part of an essential oil or extract listed above it, and gets its own line because the declaration rule requires one. Adding it by itself is possible but far less common.

Is natural limonene safer than synthetic?

No. It is the same molecule, and the declaration rules apply to it whichever way it was obtained. What changes the risk is how much of it has oxidised, which is a question about age and packaging.

Does it make skin sensitive to sunlight?

That reputation belongs to other citrus components, the furocoumarins found in some expressed peel oils, rather than to limonene itself. If phototoxicity is the worry, the oil is the line to look at, not this one.

My product says fragrance-free and lists limonene. Which is true?

Both can be. Fragrance-free usually means no fragrance compound was added, and a plant extract present for another reason can still bring limonene with it, at which point the rule requires it to be named.

Does an old bottle matter?

For this molecule, more than for most. Oxidation is what makes it a sensitiser and it proceeds once air is in the bottle. A citrus scent that has gone flat or turned sharp is the material telling you where it is.

Limonene — Ingredient Guide

How it works

Limonene is a terpene, the molecule that gives citrus peel its smell, and it turns up in conifer, herb and spice oils too. On an ingredient list it is usually not something a formulator added on its own: it arrived inside an essential oil or a plant extract named higher up, and it appears under its own name because most markets require certain fragrance allergens to be declared once they pass a set level.

It is also made industrially, and the molecule is identical either way: a nose cannot tell them apart and neither can skin, and the list does not say which you have. What it does in a product is smell, and act as a solvent, which is why citrus oils cut grease. Neither is a benefit to skin.

The evidence

Fresh limonene is a weak sensitiser. The complication is that it does not stay fresh: exposed to air and light it oxidises, and the oxidation products, peroxides formed as the molecule degrades, are considerably more sensitising than the molecule they came from. That mechanism is well documented, and it is what the declaration rule is really about.

Two things follow. The first is that the risk is not fixed by the ingredient list. The same product is a different proposition new and half-used a year later, and how it was packed matters more than where the limonene came from. The second is that formulators have an answer, antioxidants in the formula and opaque packaging that keeps air out, and you cannot tell from outside whether it was used. Most people use products containing it for years without trouble. A minority sensitised to its oxidation products will react afterwards to small amounts, and that does not reverse.

Suitability

Not an active; nothing about a skin type argues for it
A specific reason to avoid it if patch testing has named it
A general one where the barrier is broken, or the product sits on shaved or freshly exfoliated skin
A product marketed as fragrance-free can still list it, because a plant extract brought it in

Concentration

No figure. The level at which it has to be declared is set locally and has been revised, so the name tells you a threshold was crossed in that market, not how much is there. And the number that would matter most cannot be read off a label: how much of what is in the bottle is still limonene, and how much has turned into something more reactive.

Conflicts & combinations

Nothing conflicts with it. The pairing worth knowing about is already in the formula: antioxidants slow the oxidation that creates the sensitisers, which is one reason a citrus-scented oil so often carries tocopherol. The habit worth changing is keeping an opened bottle of a strongly citrus product for years. Age is the variable here, and it works in one direction.