The system primarily targets free I-131 in the air within the reactor building, though i don't understand why it wouldn't remove other varieties of isotopes as well. In any case, they are touting a possible 96.666 reduction in free air Iodine if the system performs as expected.
How the reduction of iodine impacts the total exposure rates within the building remains to be seen.
During the operation which lasted 26 minutes, exposure rates measured between 8 and 93 milliSieverts per hour at different locations within the unit. Employees were exposed to between 0.31 to 3.16 millisieverts.
Within a few days, Tepco should be following up with new radiation measurements from within the building to see how effective this makeshift ventilation system is.
Story from The Daily Yomiuri