Swellable particles for delivering a working agent to the pulmonary system comprise a plurality of biodegradable particles each formed from a polymer network, each of the plurality of biodegradable particles having a mass mean aerodynamic diameter not exceeding 5 μm, the particles being swellable by hydration to a size that is greater than 6 μm volume mean diameter, and a working agent entrapped in the polymer network of each of the plurality of biodegradable particles.
