Tradeoffs between MIs were considered using value functions to systematically evaluate characteristics with a standard unit, i.e., dollars . MIs are converted to value using exchange constants. Owing to available and robust exchange constants , comparisons were made in terms of the relative cost of materials and the relative social cost of CO2 for each material with respect to PS foam (Figure ). Regarding the cost of materials, PS foam was the least expensive. The amounts of other materials used to make food trays and takeaway containers were estimated to be 4 to 5 times more expensive. This difference translates to an additional cost of material <$0.10 more per item, which is likely negligible to the consumer with respect to the cost of the food held by the items (e.g., cost of meat ). As for the social cost of CO2, PS foam was the best choice, being 1.4 to 7.4 times less costly than the other materials. Presently, the latest estimate for the value of the exchange constant for the social cost of CO2 is 0.22 $USD/kg CO2. Scaled to the annual consumption rate of food trays and takeaway containers used in the U.S., the annual social costs of CO2 associated with PS foam food trays and takeaway containers were ~$49-98 million and ~$30-34 million, respectively. To date, no exchange constant exists for the social cost of water, though the need to determine such a number has been proposed . Nonetheless, water is a valuable natural resource ; thus, reducing its usage as much as possible can benefit society.