The concentration of chemical $\text{A}$ in ambient air is $50 ~\mu \mathrm{~g} / \mathrm{m}^{3}$ and the toxicological index limit for a threshold health effect for this chemical is $5$ $\mathrm{mg}$.
If the average breathing rate is $4 \mathrm{~L} / \mathrm{min}$, the minimum time of exposure that can lead to any health effect is $\_\_\_\_$ days (rounded off to one decimal place).
