Due to their small size, PFCs are able to permeate circulation where erythrocytes may not flow. In tiny capillaries, PFCs produce the greatest benefit, as they increase local oxygen delivery much more efficiently than would be expected from the increase in oxygen content in larger arteries. In addition, as gases are in the dissolved state within PFCs, it pO2 promotes efficient oxygen delivery to peripheral tissues. Since the mid-1980s, improvements in both oxygen capacity and emulsion properties of PFCs have led to the development of second-generation PFC-based oxygen carriers; two PFC products are currently being tested in phase III clinical trials.