Why do we not see radicals forming in the skin while sunbathing

One of the most common causes of skin cancer is excessive sunbathing. Radicals are generated in the skin during irradiation with high-intensity UV-light, e.g. while lying on a beach. These radicals react with other compounds in the skin, which may ultimately cause skin cancer. But we never see these radicals by eye, because their concentration is so minuscule. And the concentration is so small because the radicals react so fast. (Photo-ionization is discussed in Chapter 9.) 

Reaction intermediates are common in mechanistic studies of organic reactions. They are called intermediates because they react as soon as they form. Such intermediates are sometimes described as reactive because they react so fast they disappear more or less immediately . Indeed, these intermediates are so reactive, they may react with solvent or even dissolved air in solution, i.e. with other chemicals in high concentration. 

The accumulated concentration of such a fast-reacting intermediate will always be extremely low perhaps as low as 10-10 mol dm 3. 

Since the reaction of intermediates is so fast, the concentration of the radical intermediate changes dramatically, yet the concentrations of the natural compounds in the skin with which the intermediate reacts (via second-order processes) do not change perceptibly. 

How in practice, then, do we determine kinetic parameters for pseudo-order reactions such as these  

---