You just had new pads or rotors installed, and the first day of driving brings a faint hot, chemical smell from the wheels. On fresh brakes this is almost always normal - part of the break-in process. Here is what causes it and when to pay attention.
Why new brakes smell
New brake pads go through a curing process the first time they get hot, sometimes called out-gassing. The resins and binders in the fresh friction material finish curing under heat, giving off a faint burning or chemical odor for the first few drives. It is a normal part of bedding in the pads and fades once they are heat-cycled.
Manufacturing coatings burning off
New rotors also ship with a protective coating on the friction surface that must burn off, and any assembly grease that got warm contributes a whiff too. Combined with pad out-gassing, a light smell on the first drive after a brake job is expected and harmless - related to why brakes smell in general.
How long it should last
The break-in smell should fade within the first few heat cycles - a day or two of normal driving. If it is strong, pair it with a proper bedding procedure and it will settle. A persistent smell well after break-in, especially with smoke or a dragging feel, is different.
When the smell is a warning
Be concerned if the smell continues past break-in, comes with smoke, a burning-hot wheel, or a brake that drags - that points to a sticking caliper or seized slide pin, not curing. Otherwise, let the new pads and hardware bed in and the smell will pass.