Air Carrier Captain reported being notified by the flight attendants of a 'chemical/exhaust' odor after departure. The fumes dissipated after a brief period only to resume during descent. Flight Attendants received medical evaluation after the flight.

Narrative

After takeoff from ZZZ; the FA's (Flight Attendant) reported a chemical/exhaust smell and a brief period of a haze in the cabin. Both of the items soon disappeared; but we were able to briefly smell a exhaust type smell briefly in the cockpit as well. As we started the descent into ZZZ1 and the power levers retarded to idle thrust; the odor returned and was reported by the rear FA's but without a haze report this time. The odor was faintly noticeable in the cockpit and would dissipate quickly once the power levels were returned to a greater than idle power setting. After landing and upon gate arrival; I consulted with the FA's and decided the aircraft was to be written up for a smoke/odor/fume event. The FA's met with their respective Flight Service managers and were evaluated by EMT's at the airport. The EMT's instructed the FA's to seek further medical attention. At that point; the FA's left the airport for the hospital to be further evaluated. The FO (First Officer) and I decided that we did not need medical attention. Our aircraft was taken out of service and eventually cancelled due to lack of cabin crew.The aircraft had been sitting in ZZZ for 24 hours with constant rain. My guess is certain areas that get hot following application of bleed air may have steamed off latent oil and/or other fluids that may have been ingested into the packs.[I suggest] more aircraft maintenance that revolves around preventative care of the aircraft.