Table 1.

Result of independent t-test (all values in ppm)

Pollutant (ppm)	Before control			After control			MD	t	Significant (two-tailed)
	n	Mean	SD	n	Mean	SD
Benzene	32	31.98	26.08	19	4.50	3.64	27.50	5.84	0.001
Toluene	32	105.82	74.88	19	44.50	24.39	61.33	4.27	0.001
P and M-xylene	32	145.16	39.70	19	56.16	30.52	89	8.40	0.001
O-xylene	32	76.60	28.83	19	41.57	26.56	35.03	4.32	0.001

MD - mean difference, SD - standard deviation.