At the plant on the 19th the iodine in air was higher than the allowed levels for radiation workers
Iodine-131, 5.9 Bq per litre
Iodine-132, 2.2 Bq per litre
Iodine-133, 0.035 Bq per litre
Accoridng to the web the ALI for iodine-131 inhalation is 1 MBq per year.
ALI is Annual Limit of Intake, in europe it is normally set at the amount of radioactivity which would cause you to get a 20 mSv for a radiation worker.
While 5.9 Bq per litre of iodine-131 per litre might seem small it could add up, according to one company which makes air line breathing hoods, a person who is digging will use 60 litres per minute.
Now if you use 60 litres per minute of air, then it is 354 Bq per minute of iodine being inhaled, which works out in turn as 21240 Bq per hour. Now at this level a person will reach their ALI in 47 hours. This is assuming that the person is only being exposed to iodine-131 in the air.
So the use of a charcoal filter to reduce the amount of iodine that a person breathes in is likely to be a good idea. I am making the assumption that all the iodine that a person inhales is absorbed in their lungs.