Sorry for the delay.
I believe that frequency of Spirometry IS PRESCRIBED in Section 4 but NOT Section 2. Let me explain:
First to clarify what took me a few minutes to figure out: Spirometry, i.e., pulmonary testing meeting Standardization of Spirometry, are among the tests that may have to be included as part of the control program required for designated substances other than arsenic or ethylene oxide (including, of course, isocyanates) under the reg. (Section 20(4) of Designated Substances Reg.).
Section 2 of the Medical Surveillance Code (which fleshes out the clinical testing requirements of the Reg.) lists 4 points at which Sec. 20(4) testing is required:
1.Before placement of worker to position involving exposure
2. Periodically:
 At 6 month intervals for the first 2 years;
 Annually after the first 2 years;  or
 More frequently, if required by the examining physician.
3. In the event of an acute exposure requiring immediate medical attention
4. Upon exiting placement unless the most recent periodic medical examination was performed within the last 6 months 
Section 4 then talks specifically about testing for isocyanates, including Spirometry.
MY TAKE: The Section 2 intervals cover Section 20(4) testing for covered designated substances other than Isocyanates (even though they’re included in the Isocyanates Heading of the Code); Section 4 lists the frequencies for isocyanates which I read as being separate from Section 2. Otherwise, why even have a section for isocyanates’
But, yes, this is quite confusing and my interpretation is just that–an interpretation. You may want to run your Q by somebody at MOL and get their take. If you do, please let me know what they say.
Thanks for the challenging Q and I apologize for taking so long to answer it–if, in fact, I even did answer it. But I did try. . . Glenn