I found the whole text here. The context of that sentence is when the author explains proposition #11, part of the explanation of paticca-samuppada which
shows the causes and conditions of all the suffering in the world; and how, through the removal of these conditions, suffering may rise no more in the future.
About proposition #11:
Now we have reached the eleventh and last proposition: Jati-paccaya jara-maranam: "Through rebirth decay and death are conditioned." Without birth there cannot be decay and death. If we had not been born, we would not have to die, and would not be exposed to all sorts of misery. Thus rebirth is a necessary condition for decay and death, and for all other forms of misery. Hence it was said: "Through rebirth decay and death are conditioned."
Because the context is about removal of the causes and conditions of an individual's suffering in the world, "exposed" here mean vulnerable / susceptible.
Other examples of "exposed" in this meaning:
- If we go out in the rain without bringing an umbrella, we will be "exposed" to being wet.
- If we put all our money in stocks, we will be "exposed" to the risk of market crashing.
- Etc.