Consider the following data set which relates the hemoglobin levels h (kg/dL) of workers and the altitude a (km) above the sea level that the workers exposed over years:

a 0.32 0.64 1.28 1.60
h 1.15 1.10 1.05 0.95

In the regression model h = c1e^-c2a, the constant c2 is found as c2 = 0.1315. Assuming the hemoglobin concentration for the residents at the highest permanently tolerable altitude of the atmosphere is the 1/1000th of the hemoglobin concentration for the residents at sea level, compute the highest permanently tolerable altitude in kilometers.