A I R Q U A L I T Y A S S E S S M E N T
Indoor PM₂.₅ Decision Tool
Step-by-Step Scenario Walkthrough
S c e n a r i o A
PM₂.₅ = 5 µg/m³ · AQI = 15
→ All Clear
S c e n a r i o B
PM₂.₅ = 55 µg/m³ · AQI = 60
→ Action Required
EPA PM₂.₅ standard: 10 µg/m³ (24-hr) · Decision support tool, not clinical advice
Two Scenarios at a Glance
How the tool routes decisions based on PM₂.₅ and AQI inputs
Parameter Scenario A
(Low Pollution)
Scenario B
(High Pollution) EPA Standard
Indoor PM₂.₅ 5 µg/m³ 55 µg/m³ ≤ 10 µg/m³
Status vs EPA ✓Within standard ✗Exceeds by 5.5× Threshold
Outdoor AQI 15 (Good) 60 (Moderate) ≤ 50 = Good
Ventilation No action needed Keep windows closed —
HVAC needed? No Yes —check filter —
Outcome All Clear Deploy air purifier —
Scenario A · PM₂.₅ = 5 µg/m³ (Below EPA Standard)
STEP 1 Enter Indoor PM₂.₅
Reading: 5 µg/m³
EPA standard: 10 µg/m³ (24-hr)
→ Value is within the standard.
✓Within standard
STEP —Result: All Clear
PM₂.₅ of 5 µg/m³ is at or below
the 10 µg/m³ EPA standard.
No further action required.
✓No action needed
→
✓Outcome: PM₂.₅ Within EPA Standard
Indoor air quality is acceptable. No ventilation changes, HVAC intervention, or air purifier are needed at this time.
Monitor periodically and re-assess if occupancy or activity patterns change.
Scenario B · PM₂.₅ = 55 µg/m³ (Exceeds EPA Standard)
STEP 1 Enter Indoor PM₂.₅
Reading: 55 µg/m³
EPA standard: 10 µg/m³
→ Exceeds standard by 5.5×.
Proceed to investigate sources.
✗Exceeds standard
STEP 3 Indoor Sources?
Check for: cooking, candles,
incense, smoke, dusty activities,
printers, pets, fireplaces.
→ No obvious source found.
→ Check outdoor
→
⚠Action Required: PM₂.₅ is 5.5× above the EPA standard
No identifiable indoor source was found. The next step is to check outdoor air quality (AQI) to determine
whether the elevated levels are driven by outdoor infiltration —and whether ventilation should be used or avoided.
Step 5 · Check Outdoor AQI —Two AQI Scenarios
AQI = 15 · GOOD
AQI
15
Source: AirNow.gov
AQI ≤ 50 → Outdoor air is GOOD
→ Open windows to dilute indoor PM₂.₅
→ Cross-ventilate for 30–60 minutes
→ Proceed to Step 7: Open Windows
AQI = 60 · MODERATE
AQI
60
Source: AirNow.gov
AQI > 50 → Outdoor air is MODERATE
→ Do NOT open windows
→ Keep building sealed; use HVAC
→ Proceed to Step 6: HVAC Assessment
Scenario B · Steps 6–9 · HVAC Assessment (AQI = 60)
STEP 6 HVAC Operating?
Outdoor AQI = 60 (>50).
Do NOT open windows.
Is HVAC currently running?
STEP 8 Check Filter
Inspect for grey/brown
discolouration or blockage.
Replace if ≥1–3 months old.
STEP 9 Turn On HVAC
Set to recirculation mode
(not fresh air intake).
Run 1 hour, then recheck.
→ →
PM₂.₅ Recheck After 1 Hour of HVAC Operation
≤ 10 µg/m³ → All Clear — EPA standard met.
Assessment complete.
> 10 µg/m³ → Still elevated — inspect
HVAC filter condition.
Scenario B · Step 10 · Deploy Air Purifier
⚡Air Purifier Recommended
HVAC alone has not reduced PM₂.₅ to the EPA standard of 10 µg/m³. A portable HEPA air purifier is recommended
as a supplementary measure to capture fine particulate matter in the most-occupied areas.
📍Placement
Position in the most-occupied room. For
open plans, central placement maximises
coverage.
📊Sizing (CADR)
Match the purifier's CADR rating to the
room volume. 2/3 of CADR = recommended
sq ft.
🌀Speed Setting
Run on medium-high speed until PM₂.₅
drops below 10 µg/m³, then reduce to
maintain.
If levels remain persistently high despite HVAC + purifier, investigate structural infiltration pathways or consult an indoor air quality specialist.
Decision Flow Summary —Both Scenarios
Step 1
Measure
Indoor PM₂.₅
≤ 10?
All Clear ✓
Step 3
Identify
Indoor Sources?
Step 5
Check
Outdoor AQI
AQI ≤ 50
Open Windows
(Step 7) ✓
Step 6
HVAC
Assessment
Step 10
Air Purifier
Recommended
› › ›
↓ ↓ ↓
Scenario A (PM₂.₅ = 5 µg/m³ · AQI = 15)
Step 1 → All Clear. No further action needed.
Scenario B (PM₂.₅ = 55 µg/m³ · AQI = 60)
Step 1 → Step 3 → Step 5 (AQI>50) → Step 6 → HVAC → Step 10 Air Purifier
Recommended.
K E Y T A K E A W A Y S
What These Scenarios Demonstrate
01 EPA Standard is the decision threshold
The tool uses 10 µg/m³ as the EPA PM₂.₅ standard. A reading of 5 µg/m³ clears immediately; 55 µg/m³ triggers a full diagnostic.
02 AQI drives ventilation decisions
AQI ≤ 50 (Good) → open windows to dilute indoor pollution. AQI > 50 → keep sealed and rely on filtered HVAC recirculation.
03 Air purifier is recommended, not mandatory
When HVAC alone cannot bring PM₂.₅ below 10 µg/m³, a HEPA purifier is recommended as a supplementary measure.
EPA PM₂.₅ standard: 10 µg/m³ (24-hr) · AQI data: AirNow.gov · Decision support tool, not clinical advice
