Young massive stellar clusters (YMSCs) are able to modify the circumstellar medium around them thanks to the feedback produced by the powerful winds of massive stars, which inflate low-density bubbles with sizes of tens to a hundred parsecs. As the entire cavity grows in time, the regular interstellar medium is swept away, and wind advection modulates the propagation of low-energy Galactic cosmic rays (CR), potentially preventing their penetration inside the bubble. In such a situation, dense molecular clouds positioned inside the wind-blown bubble can only be ionized by CRs self-produced by sources inside the YMSC (like stellar winds). Recently, YMSCs have been suggested to be powerful sources of CRs. Hence, in this presentation, we propose to use the ionization level of clouds close to YMSC to estimate the production efficiency of low energy CRs. We specialize our calculations to a model where CR acceleration occurs at the termination shock of stellar winds, investigating how the ionization rate induced by such freshly accelerated particles differs from the one expected by the average Galactic CR flux. Finally, we focus on the specific case of the Cygnus OB2 association. We show how the combination of gamma-ray observations and measurements of the ionization level of nearby molecular clouds represents a powerful tool to constrain the properties of the CR distribution in the cluster neighborhood.