Nine of 28 carbon monoxide poisoning accidents that resulted from defective gas water heaters were caused by improperly altered safety devices coupled with dirty heat exchangers and deterioration of parts, Paloma Industries Ltd. reported Monday.

President Toshihiro Kobayashi presented a report several hundred pages long to the Ministry of Economy, Trade and Industry covering an in-house probe into carbon monoxide poisoning accidents that resulted in 21 deaths from 1985 to 2005.

The document filed by the Nagoya-based company includes measures to prevent similar accidents, such as setting in-house standards to recall defective products.