LETTER TO THE EDITOR.

A FAVORABLE COMPARISON WITH AIRCRAFT-SPECIFIC LIMITS DOES NOT ASSURE ACCEPTABILITY Chen et al. suggest that the favorable comparison of aircraft data to aircraft design standards implies acceptability. 5 The aircraft limit should be lower than that for ground-based applications; because, in operation, the aircraft environment is reduced pressure, which increases susceptibility to hypoxia. Keywords: air quality; aircraft; exposure limits; fumes; marker compounds; thermal environment EN air quality aircraft exposure limits fumes marker compounds thermal environment 1 4 4 03/02/22 20220201 NES 220201 CONFLICTS OF INTEREST The author is employed in the safety, health, and security department of a union, which represents nearly 50 000 flight attendants at 17 US airlines (Association of Flight Attendants, AFL-CIO). It is understandable that they did not cite a UFP limit in table 1 because there are no such limits currently, whether on or off aircraft.13 Notably, though, a World Health Organization guideline defines a count of >20 000 particles/cm SP 3 sp (1 h mean) as "high",14 which is considerably lower than some of the aircraft data. [Extracted from the article]