Their disadvantages include their generally high maintenance requirements, low absorption efficiency, and high operating costs.

However, absorption efficiency should not be confused with the sunlight-to-electricity conversion efficiency.

High gas velocities through these devices reduce the gas-liquid contact time, thus reducing absorption efficiency.

When insufflating cocaine, absorption through the nasal membranes is approximately 30-60%, with higher doses leading to increased absorption efficiency.

The activated carbon must be replaced periodically as it may become saturated and unable to adsorb (i.e. reduced absorption efficiency with loading).

Plant sterol absorption is quite low, particularly for stanols, for which the absorption efficiency is over 10 times lower than for the equivalent sterols.

This short contact time limits the absorption efficiency of the system.

Multiple-stage systems provide increased gas-liquid contact time, thus increasing absorption efficiency.

"The absorption efficiency of all these forms of calcium is similar, at about 30 percent," Dr. Hendler said.

Some studies, but not all, suggest that the absorption efficiency of calcium citrate is greater than that of calcium carbonate, he said.