Recommendations by health experts to deal with public health emergencies are primarily guided by the principle of "saving more lives". It is unclear whether people perceive this principle as ethically more legitimate than some other principle such as "saving more life-years". Understanding the answer to this question is particularly relevant to the allocation of scarce medical resources during public health emergencies. Different principles typically lead to different allocations, and consequently have dramatically different implications as to who survives and who dies. We fielded an online randomized controlled survey experiment in the context of scarce ventilator allocation with a demographically representative sample of US adults (n = 700) from October 22 to October 30, 2020. Participants faced hypothetical situations where they had to allocate few available ventilators among several needy patients. The experiment was designed such that the allocation decision made by a participant can be used to infer the principle in line with their personal ethical values. We interpret this inferred principle as the one that the participant perceives to be most legitimate. The treatment group, but not the control group, was provided balanced information that described the ethical dilemmas faced by experts in developing ventilator allocation guidelines. Nearly half of the participants in the control group perceive saving more lives the most legitimate principle. Despite the balanced nature of the information, the perceived legitimacy of saving more lives was 7·6 percentage points higher in the treatment group. The magnitude of this impact was particularly strong among republican-leaning participants, a subgroup that has less trust in experts according to previous research. Our findings suggest that enhancing public awareness of ethical dilemmas faced by health experts can increase the perceived legitimacy of their proposed guidelines even among those with lower trust in experts.