A limitation of their methodology was that some VOCs may have already evaporated from the oil before they began their investigation. In their report, they suggest the possibility that respiratory symptoms might have been caused by high levels of ozone or reactive aldehydes in the air, possibly produced from photochemical reactions in the oil. NIOSH did note that many of the personnel involved were not donning personal protective equipment (gloves and impermeable coveralls) as they had been instructed to and emphasized that this was important protection against transdermal absorption of chemicals from the oil.