Two people died at the scene of the explosion at the Schiattarella Brothers Pyrotechnics factory in Ponte Ricco, an industrial zone in Giugliano, Il Mattino reported.
One of the owners, 55-year-old Raffaele Schiattarella, died of his injuries in hospital. His brother, Salvatore, is in Cardarelli hospital in Naples.
Two others were hospitalized in Pozzuoli.
A fire broke out after the blast, according to media reports.