Every contaminant Purity Guardian tracks, explained in plain English — with the EPA, ASHRAE, and WHO standards behind our recommendations.
PM2.5 refers to airborne particles smaller than 2.5 micrometers — roughly 1/30th the width of a human hair. These ultrafine particles are invisible to the naked eye and can remain suspended in indoor air for hours. They're produced by combustion (cooking, candles, fireplaces), vehicle exhaust, industrial emissions, and even reactions between other pollutants in the air.
Cooking with gas or high heat, burning candles or incense, tobacco smoking, fireplaces and wood stoves, attached garage vehicle exhaust, printshop/industrial off-gassing, HVAC systems with dirty filters, and outdoor pollution that infiltrates through windows and cracks.
| Standard | Safe Level | Notes |
|---|---|---|
| EPA NAAQS | Annual avg: 9 μg/m³ 24-hr: 35 μg/m³ |
National Ambient Air Quality Standard. The 9 μg/m³ annual limit was tightened in 2024 from the previous 12 μg/m³. |
| WHO (2021) | Annual: 5 μg/m³ 24-hr: 15 μg/m³ |
More stringent global guideline reflecting new health evidence. WHO acknowledges no safe threshold exists. |
| ASHRAE 62.1 | Outdoor air ≥ 5 μg/m³ | Minimum outdoor air quality for ventilation design. Lower outdoor PM2.5 means less filtration required. |
Minimum MERV 13 rated for capturing PM2.5. Replace every 3–6 months.
Exhaust fans reduce particle concentration by up to 70% during cooking.
When outdoor AQI is poor, close windows and use a HEPA air purifier.
One candle can elevate PM2.5 to unhealthy levels within 30 minutes.
Carbon dioxide is a colorless, odorless gas produced naturally by human respiration, combustion, and decomposition. In indoor environments, it's a reliable proxy for air staleness — when CO₂ rises, it means exhaled breath is accumulating and fresh outdoor air isn't diluting it. High CO₂ itself isn't acutely toxic at indoor levels, but it signals poor ventilation which lets all other indoor pollutants build up.
Human breathing (the primary driver — ~40 ppm increase per person per hour in a sealed room), combustion appliances (gas stoves, furnaces, water heaters), attached garages with running vehicles, restaurants and coffee shops with active occupancy, and any poorly ventilated enclosed space with multiple occupants.
| Standard | Level | Notes |
|---|---|---|
| ASHRAE 62.1 | ≤ 1,000 ppm | Maximum recommended indoor CO₂ for occupant comfort and cognitive performance. Beyond this, air is measurably stale. |
| OSHA | 8-hr TWA: 5,000 ppm | OSHA's permissible exposure limit. Not health-protective for comfort — this is an industrial threshold. Indoor residential comfort targets are much lower. |
| NIOSH | REL: 1,000 ppm | National Institute for Occupational Safety and Health recommends ≤ 1,000 ppm for 8-hour work shifts to maintain cognitive function. |
Open windows when outdoor air is clean. Increase HVAC outdoor air damper setting above minimum.
Ensure outside air dampers are functional and sized for current occupancy loads.
Wall-mounted CO₂ sensors with display let you see real-time air freshness. Target below 800 ppm.
Gas appliances and fireplaces can spike CO₂ and CO together — always use ventilation.
VOCs are a broad class of chemicals that easily become vapors at indoor temperatures. They're emitted by a massive range of everyday products — paints, adhesives, cleaning supplies, cosmetics, printers, furniture, carpet, and building materials. There's no single "VOC" — there are hundreds. Some, like formaldehyde and benzene, are known carcinogens; others are just unpleasant. Total VOC (TVOC) is a sum metric; individual compounds matter more for health risk.
Paints and finishes (especially oil-based and lacquers), adhesives and caulks, cleaning and disinfecting products, printers and copiers, pressed wood furniture and cabinets (formaldehyde), carpet and upholstery, air fresheners and candles, personal care products (nail polish, hairspray), hobby supplies (solvents, adhesives), and attached garage vehicle exhaust seeping indoors.
| Standard | Level | Notes |
|---|---|---|
| EPA (Indoor Air) | TVOC < 500 μg/m³ (baseline, no adverse effects) |
Below this level, most people experience no sensory irritation. Above 500 μg/m³, irritation and discomfort increase. Above 3,000 μg/m³, significant neurological effects are possible. |
| WHO (2009) | TVOC range: 100–3,000 μg/m³ (varies by compound) |
WHO notes individual VOC compounds have specific limits; TVOC alone doesn't capture compound-specific risk. Formaldehyde is regulated separately at 80 ppb (8-hour average). |
| California CDPH | Formaldehyde < 22 ppb (residential & school) |
California's Section 01350 standard is the most stringent in the U.S. Requires low-emitting materials in new construction and major renovations. |
Choose Greenguard-certified paints, finishes, and adhesives. Look for "low-emission" labels on furniture.
Lower temperatures reduce VOC off-gassing. Keep indoor temp below 75°F and RH between 30–50%.
Open windows during painting, cleaning, or printing. Run exhaust fans. Air out new furniture outdoors if possible.
HEPA alone doesn't capture VOCs — you need activated carbon (charcoal) filtration for effective VOC removal.
Radon is a radioactive gas produced naturally by the decay of uranium in soil and bedrock. It seeps up through cracks in foundations, gaps around pipes, and porous concrete. It accumulates in basements and ground-floor spaces. The only way to know if your building has elevated radon is to measure it — it's completely invisible and odorless. EPA estimates radon causes ~21,000 lung cancer deaths per year in the U.S., making it the second leading cause of lung cancer overall.
Cracks in concrete foundations and floors, gaps around pipes and cables where they penetrate the floor, sumps and drain tiles, porous concrete block walls, joints between walls and floors, and soil gas migration through the building's foundation. Newer, more airtight buildings can trap radon more effectively than older leaky structures.
| Standard | Level | Notes |
|---|---|---|
| EPA Action Level | 4.0 pCi/L | EPA recommends radon mitigation if long-term average is ≥ 4.0 pCi/L. This represents roughly a 1 in 1,000 lifetime cancer risk from indoor exposure. |
| WHO (2009) | 100 Bq/m³ (≈ 2.7 pCi/L) | WHO recommends 100 Bq/m³ as the action threshold, acknowledging lower levels still carry risk. EPA's 4.0 pCi/L ≈ 148 Bq/m³ is more permissive. |
| ASHRAE 62.1 | 4.0 pCi/L as reference | References EPA's action level for residential radon. ASHRAE notes that radon reduction is a building-specific mechanical solution, not a general ventilation rate requirement. |
Use a certified long-term test kit (90+ day average) or hire a state-certified radon measurement professional.
A radon mitigation system (active soil depressurization) can reduce levels by 50–90%. Cost: $800–$2,500.
Use caulk and expanding foam to seal cracks in concrete floors and walls. Reduces radon entry by 20–30%.
Properly vent crawl spaces and install sub-membrane ventilation. Avoid sealing basement floors without a vent path.
Ozone (O₃) is a molecule of three oxygen atoms — a powerful lung irritant and oxidant. There's "good" ozone high in the stratosphere (the ozone layer), and "bad" ozone at ground level, which is a component of smog. Indoor ozone primarily comes from certain air purifiers (those using corona discharge or UV lamps), laser printers, and office equipment. Unlike other pollutants where some exposure is unavoidable, indoor ozone is often generated inside your building by equipment you'd never think to question.
Some "air purifier" brands using ionizer or electrostatic precipitation technology, office equipment (laser printers, copy machines), UV sterilizers and some HVAC UV-C lamps, industrial processes, and outdoor smog that infiltrates through ventilation. Some ozone-generating devices are marketed with misleading claims about "fresh air" — consumer Reports has documented this extensively.
| Standard | Level | Notes |
|---|---|---|
| EPA NAAQS | 8-hour: 0.070 ppm | National Ambient Air Quality Standard. Ground-level ozone above this over 8 hours is considered unhealthy for sensitive groups. |
| California ARB | 8-hour: 0.060 ppm (more stringent) |
California's standard is more protective. ARB also limits ozone-generating devices sold in California. |
| UL 867 (Electrostatic) | ≤ 50 ppb from devices | Underwriters Laboratories standard for electrostatic air cleaners. Devices must not emit more than 50 parts per billion of ozone. |
Check your air purifier. Any "ionizer," "electrostatic precipitator," or "ozone generator" on the device = a source of indoor ozone.
Locate laser printers in well-ventilated areas. Ensure equipment rooms have exhaust ventilation.
True HEPA filtration removes particles without generating ozone. Look for CADR-certified units with HEPA filters.
During high outdoor ozone days (ozone action days), minimize ventilation and run air purifiers with HEPA filters.
Mold is a fungus that grows when moisture meets organic material (drywall, wood, carpet, paper) over a sustained period. It's not a single contaminant — it's a biological growth that produces allergens, mycotoxins, and irritants. The EPA and CDC state there is no practical way to eliminate all mold and mold spores indoors; the goal is to control moisture. Mold growth typically begins when relative humidity exceeds 60% for more than 24–48 hours, especially in the 68–86°F temperature range — the "mold growth zone."
Water leaks from roofs, plumbing, or foundations, condensation on cold surfaces (windows, walls, pipes), HVAC systems with standing water or dirty coils, humidifiers used without monitoring, basements and crawl spaces with high moisture, clothes dryers vented indoors, cooking without exhaust ventilation, and any flooding event where materials didn't dry within 24–48 hours.
| Standard | Threshold | Notes |
|---|---|---|
| EPA / CDC | Indoor RH: 30–60% | Both agencies recommend keeping indoor relative humidity between 30–60% to discourage mold growth. Below 30% causes other issues (dry skin, static); above 60% significantly increases mold risk. |
| ASHRAE Standard 62.1 | Control moisture at source | Ventilation standard focuses on moisture control as a primary mold prevention strategy. Requires dehumidification when outdoor humidity is high. |
| NIOSH | Moisture < 0.005 (water activity) | NIOSH defines mold growth potential as a material moisture content equivalent. ASHRAE applies this through a simplified humidity threshold. |
Any water intrusion should be dried within 24–48 hours. Mold doesn't need much — 24h above 60% RH is enough.
Use dehumidifiers in basements, bathrooms, and laundry areas. Target 30–50% for comfort and mold prevention.
Run exhaust fans during and 20 min after showering, cooking, and laundry. Ensure dryer vents exhaust outdoors.
Clogged A/C condensate drains are a major mold source inside HVAC systems. Inspect and clean seasonally.
Relative humidity (RH) is the percentage of moisture in the air relative to the maximum the air can hold at that temperature. At 70°F, air at 80% RH contains roughly twice the water vapor as air at 40% RH. Humidity interacts with all other contaminants: high humidity accelerates VOC off-gassing, makes mold growth more likely, and worsens PM2.5 retention; low humidity causes respiratory irritation and static electricity. The 30–60% range is the EPA/ASHRAE comfort zone — 30–50% is optimal for most people.
Showering, cooking, and laundry (moisture sources), basement moisture from foundation water intrusion, HVAC systems without proper dehumidification, humidifiers set too high (especially in winter), climate zone (humid vs. arid regions), occupant density, and outdoor weather conditions. In winter in cold climates, indoor RH naturally drops because cold outdoor air holds less moisture — humidifiers become necessary.
| Standard | Level | Notes |
|---|---|---|
| EPA / CDC Recommendation | 30–60% RH | The recommended indoor range. Below 30% causes respiratory and skin irritation; above 60% promotes mold, dust mites, and structural damage. |
| ASHRAE 55 (Thermal Comfort) | 30–65% RH (comfort zone) | ASHRAE Standard 55 defines acceptable humidity for occupant comfort in conditioned spaces. The 65% upper limit protects against mold and moisture damage. |
| NIST / GSA (Federal Buildings) | 30–50% RH recommended | Federal building guidelines lean toward the more conservative 30–50% range, prioritizing air quality over the upper comfort boundary. |
Digital hygrometers are inexpensive (under $20). Place in living areas and bedrooms; check daily during extreme weather.
If indoor RH drops below 30% in heating season, use a humidifier. Aim for 30–45% — more is not better.
Basements and humid climates: run a dehumidifier to maintain below 60% RH. Set to 45–50% for optimal comfort.
An energy-recovery ventilator (ERV or HRV) can maintain ventilation while controlling humidity — better than opening windows in humid climates.
Temperature influences indoor air quality in multiple ways: higher temperatures accelerate VOC off-gassing from materials (up to 2× the rate at 86°F vs. 68°F), raise the mold growth threshold (mold can grow at lower humidity in warmer conditions), and directly affect occupant cognitive performance. Studies at Columbia University and Lawrence Berkeley National Lab have demonstrated measurable cognitive score differences at temperatures above 77°F versus below 71°F in office settings.
HVAC system settings and equipment age, direct sunlight through windows (can raise room temp by 10°F+), occupancy density (each person generates ~100 watts of heat), appliances and electronics, cooking activity, and outdoor weather conditions. Uncontrolled temperature swings create condition changes that affect all other IAQ parameters.
| Standard | Range | Notes |
|---|---|---|
| ASHRAE Standard 55 | 68–76°F (winter) 72–80°F (summer) |
Thermal comfort standard for occupied spaces. The range accounts for clothing insulation and metabolic rate. Lower end is winter (more clothing); upper end is summer. |
| EPA / CDC (Workplace) | 65–76°F for general offices | Federal workplace guidelines for indoor temperature. Below 65°F can cause circulation issues; above 76°F causes thermal discomfort and reduced productivity. |
| NIOSH (Workplace) | 68–76°F (ideal) | NIOSH recommends 68–76°F with 20–60% RH for optimal worker health and productivity. Outside this range increases health complaints significantly. |
This range satisfies both ASHRAE comfort and cognitive performance research. Every degree above 76°F reduces measurable performance.
South and west-facing windows with blinds or shades prevent significant temperature spikes in afternoon.
Keep bedroom at 65–68°F for optimal sleep quality. Use a programmable thermostat to cool bedrooms automatically at night.
Use Purity Guardian's continuous monitoring to identify rooms that run significantly hotter — often indicates poor airflow or equipment issues.
Professional IAQ assessment reports scored against the standards referenced above — delivered to your inbox in minutes.