Sussex Police said: "A small number of people reported symptoms including sore eyes and vomiting, two of whom were taken to hospital. They have both since been discharged, and advised to go home, wash their clothes and have a shower.
"Anyone else affected is advised to do the same - and use copious amounts of water to wash your eyes.
"We'd like to thank everyone for their patience and understanding during the incident. We are following up a number of lines of enquiry and the investigation into what happened is ongoing."
A number of people on Twitter said they were evacuated from the seafront in the Sussex resort, including around the pier.
People in the surrounding area were told to shut their windows and doors.
Earlier in the day, East Worthing and Shoreham MP Tim Loughton wrote on Twitter the investigation was in response to reports of "bathers affected by some sort of chemical type irritants".
Christine Johnston from the Kite, Surf and Sup Co in Worthing told Sky News: "Two beach patrol guys on a buggy came by my surf shop. They had their faces covered and they told us to leave immediately.
"The beach patrol guys wouldn't comment on why they were asking us to leave. We stopped our lesson and packed up our kite, which took 20 minutes and then we said we'd meet our students at a cafe further east.
"It was quite bizarre and sounded really dramatic - we've cancelled all our lessons for the rest of the day."
The fire service says a cordon had been set up along a near 2.7km (1.6mile) stretch of Marine Parade and Brighton Road, the roads running parallel to the beach - which has since been removed.
But at least three sources told Sky News the road and pier are open and numerous other Twitter users have said there is no "lockdown".
One Sussex Police officer has tweeted a picture of a cordon along New Parade.
A Worthing Fire Station tweet, just before 2pm, said: "Emergency services are currently responding to a hazardous material incident in Worthing.