In ASHRAE 62.1, the target indoor air quality (IAQ) is described as the IAQ goal you would expect to have inside the space if the outdoor air used for ventilation were at the specified target outdoor air quality. In other words, the indoor goal is tied to the quality of the outdoor air; once you bring in that outdoor air (and treat it as needed with filtration and control), the indoors should reflect that target level. This makes the outdoor environment the reference point for what we’re aiming to achieve indoors, rather than basing the target on internal-only thresholds. The other ideas don’t define what target IAQ is: occupant surveys relate to perceived IAQ rather than the objective target; maximum allowed contaminant levels are regulatory limits and not the design target; and while IAQP or other design strategies are methods to meet the target, they describe how you achieve the target rather than what the target itself is.

Space-by-space ventilation means you determine and deliver outdoor air based on the needs of each individual space, not a single rate applied to the whole building. Each space has its own minimum outdoor air requirement determined by how many people use it, how big it is, and what it’s used for. The building’s total ventilation is then the sum or appropriate distribution of these per-space requirements, with controls that can adjust to actual occupancy (demand-controlled ventilation). This ensures rooms with higher occupancy or more likely to generate contaminants get enough fresh air, while smaller or less-used spaces aren’t over-ventilated, improving IAQ and energy efficiency. Relying on one central rate would mis-match ventilation to many spaces’ needs; spaces without windows still require ventilation, and rates aren’t set by outdoor temperature in this approach, though temperature can influence energy-saving strategies.

The key idea is that the outdoor air required to meet IAQ must actually reach the space, but the air that ends up in the zone is a mix of outdoor and recirculated air, with distribution losses in the system. To guarantee the zone gets the necessary amount of outdoor air, the zone supply must be larger than the outdoor air requirement. The standard sets the minimum zone supply outdoor-air rate as Vpz-min = 1.5 × Voz, meaning you must design the zone’s supply to deliver at least 50% more outdoor air than the basic outdoor-air requirement. For example, if the outdoor-air requirement Voz is 100 cfm, you design the zone supply to provide at least 150 cfm of outdoor air, so after mixing with recirculated air the zone effectively receives the required 100 cfm.

The distinction being tested is how outdoor-air requirements are allocated. In space-by-space ventilation, the outdoor-air rate is specified for and applied to each individual space, so different rooms can have different rates based on their use and occupancy. In system-wide ventilation, a single central outdoor-air rate is established for the whole building and distributed through the system to all spaces. So the statement that space-by-space ventilation applies rates to individual spaces while system-wide uses a central rate accurately describes the difference. The other ideas don’t fit: system-wide is not inherently more or less efficient in all cases, and space-by-space ventilation does consider occupancy because those per-space rates often depend on how many people use the space. System-wide ventilation isn’t limited to local exhaust; it involves the central supply and distribution to the spaces, not just exhaust only.

Occupancy-specific addenda to ASHRAE 62.1 exist to tailor ventilation and IAQ requirements to the needs of particular occupancies. These addenda provide updates and clarifications for settings such as healthcare facilities and schools, recognizing that patient rooms, waiting areas, classrooms, and other spaces have different exposure risks and operation patterns. For healthcare, the focus is on controlling airborne contaminants and isolation needs; for schools, the emphasis is on classroom ventilation rates, occupancy schedules, and common pollutant sources in educational environments. Because of this, the updates aren’t limited to residential buildings or to commercial kitchens; they cover a range of occupancies, including those mentioned and others. So the correct understanding is that there are occupancy-specific addenda with examples like healthcare and schools.