The going theory is that event organizers skimped on lighting costs by using UV lights intended for sanitization, not for entertainment, causing burns to the eyes and skin. The eye condition, photokeratitis, is better known as "snow blindness" or "welder's flash", as it more typically affects people who haven't worn proper eye protection while welding or while exposed to sunlight reflected from ice and snow.
Several attendees reported having to seek emergency medical treatment after experiencing excruciating eye pain and vision problems, and tweet threads began circulating giving various other ApeFest attendees advice on recovering from the painful condition.
Bored Ape creator Yuga Labs belatedly issued a tweet two days after the incident, claiming only a small fraction of attendees had experienced "eye-related issues", but encouraging anyone with symptoms to "seek medical attention just in case".