In many cap-and-trade systems, organizations which do not pollute may also participate, thus environmental groups can purchase and retire allowances or credits and hence drive up the price of the remainder according to the law of demand.
In many cap-and-trade systems, organizations which do not pollute (and therefore have no obligations) may also participate in trading. Thus environmental groups may purchase and retire emission permits and hence drive up the price of the remaining permits according to the law of demand.