R (3.1.2) sub-setting code related to "NA" on Windows not working!

My AirQuality[4,1] is equal to 18 and AirQuality[5,1] is equal to NA.
When I type the following

is.na(AirQuality[5,1])[1] FALSE> AirQuality[5,1][1] NA68 Levels: 1 10 108 11 110 115 118 12 122 13 135 ... NA> is.na(AirQuality[4,1])[1] FALSE> AirQuality[4,1][1] 1868 Levels: 1 10 108 11 110 115 118 12 122 13 135 ... NA> 

For is.na(AirQuality[5,1]) I am expecting a TRUE (but False is the output from R). Can you please help. Or Let me know what I am doing wrong!
RegardsSarada