Why did EPA select the twenty percent concentration cut-off value for aqueous ammonia?

The list of regulated substances in 40 CFR §68.130 includes aqueous ammonia that is at a concentration of 20 percent (by weight) or greater. Why did EPA select 20 percent as the concentration cut-off value?

Commonly used commodity solutions of ammonia (which mean the bulk shipments, not bottles of ammonia you can buy in the supermarket which are considerably more dilute) usually contain at least 20 percent ammonia by weight. The 20 percent concentration cut-off was proposed for regulation so that these solutions would be covered by the risk management program regulations (58 FR 5110; January 19, 1993). EPA did not revise the concentration cutoff in the final rule because such solution exceeded the volatility criterion of the final rule (59 FR 4488; January 31, 1994).