Why do new jeans smell like chemicals?
Usually finishing resins and dyes. Most new jeans carry formaldehyde-based resins used for wrinkle resistance and crease setting, plus sulfur dyes in dark washes and starch-based sizing left over from the loom. The smell is common, it usually fades, and one wash removes most of it.
What is actually on the fabric
The main culprit is the durable-press finish. Mills treat denim with resins so jeans hold a crease and resist wrinkling on the store shelf, and those resins off-gas when the garment is new. Dark rinse jeans add a second source: sulfur dyes, which have a distinct eggy, chemical edge that is strongest before the first wash. Then there is sizing, the starch coating applied to warp yarns so they survive weaving, which contributes a musty note. All three fade with washing and air.
Is it dangerous?
Formaldehyde levels in finished garments are regulated in most markets, and the amounts that remain are low. The smell is noticeable well below harmful levels, which is why strong-smelling jeans are usually still fine. Still, wash before wearing and ventilate the room while they air out. If the smell is strong enough to irritate your eyes or throat, or it survives three washes, return them. That is outside normal.
How to get rid of the smell
Wash inside out in cold water with a cup of white vinegar, then line dry outside if you can. The vinegar helps release the sizing and takes the edge off the dye smell, and fresh air does the rest. Repeat once if needed. Skip perfumed softeners: they layer fragrance over the problem instead of removing it, and they coat the fibers in a way that traps the remaining odor underneath.