Formation of carbonic acid from breath?
It is possible that carbonic acid will form; however, only in small amounts because the equilibrium of the hydration of $\ce{CO2}$ to $\ce{H2CO3}$ lies mainly on the educt side. As more $\ce{CO2}$ is soluble in cold water, more carbonic acid will be present in the solution at lower temperatures.

$$\ce{CO2 + H2O \rightleftharpoons H2CO3}$$

At $25~\mathrm{^\circ C}$ and in pure water, the quotient of the concentrations is $[\ce{H2CO3}]/[\ce{CO2}] \approx 1.7 \times 10^{-3}$, so most carbon dioxide remains physically dissolved in water. This makes carbonic acid - despite a $\mathrm{p}K_\mathrm{a}$ of $3.6$ for the first dissociation step - a fairly weak acid. It is not a dangerous compound, otherwise it would not be used in carbonated drinks. It is also a natural constituent of blood, where it acts as a pH buffer. In red blood cells, it is produced as an intermediate by the enzyme carbonic anhydrase, which catalyzes the hydration of carbon dioxide.