From the chairman of the US Nuclear Regulatory Agency:

The Congressional testimony by Gregory Jaczko, the chairman of the commission, was the first time the Obama administration had given its own assessment of the condition of the plant, apparently mixing information it had received from Japan with data it had collected independently.

Mr. Jaczko’s most startling assertion was that there was now little or no water in the pool storing spent nuclear fuel at the No. 4 reactor of the Fukushima Daiichi Nuclear Power Station, leaving fuel rods stored there exposed and bleeding radiation into the atmosphere.

As a result, he said, “We believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures.”

Well, what constitutes 'extremely high' radiation levels? I've cobbled together (from various sources) the radiation levels present around the Fukushima nuclear plant disaster. These readings span a few days, and are likely to be varying over time. In other words, this is a very imperfect sense of what's going on—but real data that we can chew on (and use to digest empty superlatives like 'extremely').

LocationRadiation Level (microsieverts per hour)
25km / 15 miles Away80
Helicopter 1000 ft above4130
Fukushima Gate10,000
Helicopter 300 ft above 87,700

The levels of radiation have a huge range—reflecting a geometric decrease in radiation levels as one gains distance from the sources of gamma radiation at the plant. Micro-sieverts aren't a particularly intuitive unit of measure. So, what I've done next is use a table of radiation doses, and their known health effects to help us make practical sense of what these numbers mean. I am only considering here the acute effects of radiation exposure, not the long-term concequences (like increased cancer risk). Nor am I considering the unique biological effects of some of the released isotopes (like radioactive Iodine). I am only considering here the health effects of the high energy (gamma and x-ray) radiation being emitted from the plant.

For this sort of radiation injury, the health effects occur after a certain cumulative threshold of radiation exposure. Putting it more simply, the radiation comes at a certain rate per hour (how fast the faucet is running); the problems come when a certain volume of radiation is absorbed (buckets of various sizes are filled).

First, my health risk table:

Health effectDose of radiation (in Sieverts)
Eye Injury0.2
Sterilization (male)0.3
Nausea / Vomiting1
Bone Marrow Suppression2
Sterilization (Female, Temporary)3
Bone Marrow Ablation (Permanent)7
Loss of gut integrity10
Fatal Brain injury20

In bold are radiation doses (and health effects) considered to be fatal.

Now, let's blend the two tables, calculating how many days would one have to spend at one of these places around the damaged nuclear plant in order to receive a radiation dose sufficient to cause each of these health effects:

Days to Injury (Hours) [Years]Tokyo15 miles AwayHelicopter 1000 ft aboveFukushima GateHelicopter 300 ft above
Eye Injury[158 yrs] (2.3h)
Sterilization (male)[238 yrs] (3.4h)
Nausea / Vomiting[792 yrs]520.810.14.20.5 (11.4h)
Bone Marrow Suppression[1584 yrs]1041.720.28.31.0 (22.8h)
Sterilization (Female, Temporary)[2377 yrs]1562.530.312.51.4 (34.2h)
Bone Marrow Ablation (Permanent)[5545 yrs]3645.870.629.23.3 (79.8h)
Loss of gut integrity[7922 yrs]5208.3100.941.74.8 (114h)
Fatal Brain injury[15844 yrs]10417201.883.39.5 (228h)

(For the highest recorded radiation levels—in the helicopter only 300 feet above the reactor—I've included the time of exposure necessary for a health effect in hours as well, as days are too long of a measure. Likewise, for Tokyo—where values have hovered around twice the level of background radiation, days is an absurd measure; instead I've calculated in years.)

So, when the NRC chairman says 'impact the ability to take corrective measures', this is what he means. Again, the pilots and the Fukushima Fifty—deserve our deepest respect for accepting these risks.

Updated in the AM:
A bit more information has been released, the radiation doses absorbed by the helicopter pilots on a mission:

The choppers actually did so at a height of less than 300 feet, but their 10 crew members suffered no health problems with less than 60 millisievert [0.06 Sv] aof radiation measured from them after decontamination, against 100 millisievert [0.1 Sv] to which they can be exposed in an emergency mission, SDF generals said.

The limit of 0.1 Sv seems to reflect the risk of corneal damage starting roughly at that point. Remember, these are lifetime, cumulative dose thresholds, for the given health effects. These pilots have to be cautious for the rest of their lives about radiation exposure—including medical imaging exposures, primarily CT scans.