FYI...did as you suggest and added the Application code...
Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)

'This line of code clears the Activity column once a new chemical is selected from the select chemical drop down

Application.EnableEvents = False
    If Not Intersect(Target, Target.Worksheet.Range("C4")) Is Nothing Then Call ClearColumn
               
    If Range("C4").Value = "Lead" Then Call CheckForLead
Application.EnableEvents = True
End Sub
Now no message boxes at all appear even if Lead appears in cell C4 - this strange issue has happened at lot - I change the code a lil bit, and it stops working - have to go back to
the original workbook to get it going again...I'm stumped!!!
Best, SR