Air treatment devices with use-up indicators

An air treatment chemical dispensing system, comprising:

• a substrate bearing a volatile air treatment chemical capable of being dispensed from the substrate when the substrate is heated,
• an indicator unit holding a volatile indicator chemical separate from the air treatment chemical such that the indicator chemical is capable of being dispensed from the unit when the unit is heated;
• and
• a heater positioned proximate the substrate and more remote from the indicator unit than the substrate;
• wherein the system is configured such that an extent of dispensing of the air treatment chemical can be indicated by a visible cue whose appearance results from the dispensing of the indicator chemical.

An air treatment chemical dispensing system, comprising:

• a substrate bearing a first volatile air treatment chemical capable of being dispensed from the substrate when the substrate is heated, and
• an indicator unit holding a volatile indicator chemical separate from the air treatment chemical such that the indicator chemical is capable of being dispensed from the unit when the unit is heated;
• wherein the system is configured such that an extent of dispensing of the air treatment chemical is indicated by a visible cue whose appearance results from the dispensing of the indicator chemical;
• and
• wherein the indicator unit also holds a second volatile air treatment chemical capable of being dispensed from the unit when the unit is heated.