Every Tamron lens has to a greater or lesser extent seals or to prevent the penetration of moisture, dust and dirt. The professional Tamron SP G2 lenses have no less than 9 seals in the lens and also a Fluorine coating on the front lens that repels moisture and dirt. With older Tamron lenses, which do not have such seals, it can happen that dust gets inside the lens by zooming. (With a Tamron 150-600mm, no less than 1.5 liters of air is sucked in when zooming). A bit of dust can't hurt and in 99% of the cases it's not visible in the picture. 

In case of a large dust accumulation between the lens parts, a professional cleaning by the Tamron service department is the only solution.