Table 5.

Weighted arithmetic means [AM(w)] for airborne benzene level by decade

Decade	No. of measurements	AM(w), ppm	SD, ppm	p∗
1977–1979	12	18.00	11.79	A
1980–1984	2,289	50.35	26.83	B
1990–1994	305	2.78	3.57	C
1995–1999	294	0.09	0.23	D
2000–2004	3,260	0.69	1.91	D
2005–2009	6,211	4.32	12.55	D
2010–2013	3,358	4.47	17.22	D
Total	15,729	10.20	22.55	<0.0001

AM(w), weighted arithmetic mean; SD, standard deviation.

^∗Multiple mean comparison t test; different letters indicate significant differences.