  We present a systematic approach to effectively evaluate potential risk cost
caused by exposure to solar proton events (SPEs) from solar flares for the
airline industry. We also evaluate associated health risks from radiation, to
provide relevant alternative ways to minimize economic loss and opportunity.
The estimated radiation dose induced by each SPE for the passengers of each
flight is calculated using ExoKyoto and PHITS. We determine a few scenarios for
the estimated dose limit at 1 and 20mSv, corresponding to the effective dose
limit for the general public and occupational exposure, respectively, as well
as a higher dose induced an extreme superflare. We set a hypothetical airline
shutdown scenario at 1mSv for a single flight per passenger, due to legal
restrictions under the potential radiation dose. In such a scenario, we
calculate the potential loss in direct and opportunity cost under the
cancelation of the flight. At the same time, we considered that, even under
such a scenario, if the airplane flies at a slightly lower altitude (from 12 to
9.5km: atmospheric depth from 234 to 365g/cm$^{2}$), the total loss becomes
much smaller than flight cancelation, and the estimated total dose goes down
from 1.2 to 0.45mSv, which is below the effective dose limit for the general
public. In case of flying at an even lower altitude (7km: atmospheric depth
484g/cm$^{2}$), the estimated total dose becomes much smaller, 0.12 mSv. If we
assume the increase of fuel cost is proportional to the increase in atmospheric
depth, the increase in cost becomes 1.56 and 2.07 for the case of flying at 9.5
km and at 7 km, respectively. Lower altitude flights provide more safety for
the potential risk of radiation doses induced by severe SPEs. At the same time,
since there is total loss caused by flight cancelation, we propose that
considering lower flight altitude is the best protection against solar flares.
