Using countif across two sheets

Hello,

I have a range of pollution levels that I want to compare to air quality standards. Data for the pollution level is one spreadsheet, the standards for it to be compared to are in another. I am using the following command: =COUNTIF(F6:F153,">='[Tox standards.xlsx]AEGLs'!$D$24'") but it is returning 0 when it should report all the data as exceeding the standard. 

What am I doing wrong please?

Thanks.

Simon.