 nitrous acid , a component of photochemical smog and a common indoor air pollutant , may reach levels of 100 ppb where gas stoves and unvented portable kerosene heaters are used . 
 nitrous acid is a primary product of combustion and may also be a secondary product by reaction of nitrogen dioxide with water . because the usual assays for nitrogen dioxide measure several oxides of nitrogen ( including nitrous acid ) together , previous studies of indoor nitrogen dioxide may have included exposure to and health effects of nitrous acid . to assess the respiratory effects of nitrous acid exposure alone 
 , we carried out a double - blinded crossover chamber exposure study with 11 mildly asthmatic adult subjects . 
 each underwent 3-hr exposures to 650 ppb nitrous acid and to filtered room air with three 20-min periods of moderate cycle exercise . 
 symptoms , respiratory parameters during exercise , and spirometry after exercise were measured . a statistically significant decrease in forced vital capacity was seen on days when subjects were exposed to nitrous acid . 
 this effect was most marked at 25 min and 85 min after exposure began . 
 aggregate respiratory and mucous membrane symptoms were also significantly higher with nitrous acid . we conclude that this concentration and duration of exposure to nitrous acid alters lung mechanics slightly , does not induce significant airflow obstruction , and produces mild irritant symptoms in asthmatics.imagesfigure 1 . 
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