Tar smells strong because it contains volatile smoky compounds that slowly evaporate into the air. The smell varies by source, with wood tar, pine tar, coal tar, and road materials all having different odours.
Tar smell is not a single smell. Pine tar can smell smoky, resinous, leathery, and campfire-like. Birch tar may smell smoky and sharp. Coal tar is often heavier, more chemical, and more industrial. Road materials can smell hot, oily, and bituminous, especially in warm weather. People call all of these smells tar, but they come from different mixtures of compounds.
The strength of the smell comes from volatile organic substances released by the tar. When wood or coal is heated, some of the resulting compounds remain trapped in the thick liquid. They continue to evaporate slowly after the tar is made. Heat, fresh application, and large exposed surface area can make the smell much stronger.
This is why a freshly opened tin of pine tar can fill a workshop quickly, while an old tarred fence may only smell noticeable in sunlight or rain. Tar does not always become odourless when it dries. Some traditional tar finishes keep a faint smell for a long time, especially indoors or in enclosed storage.
A strong tar smell is not proof that something is unsafe, but it is a reason to be sensible. Ventilation, product instructions, and the exact type of tar matter. Coal-tar and creosote-related materials deserve much more caution than a heritage wood-tar finish used outdoors.