Sterling Chart

Detailed Explanation

The Sterling Chart was first introduced in the paper "Criteria for Human Exposure to Humidity in Occupied Buildings" by E. M. Sterling, A. Arundel and T. D. Sterling, published in ASHRAE Transactions in 1985, and became widely known through the article "Indirect Health Effects of Relative Humidity in Indoor Environments" by A. V. Arundel, E. M. Sterling, J. H. Biggin and T. D. Sterling, published in Environmental Health Perspectives in 1986. The chart combines the effects of relative humidity on bacteria, viruses, mold, mites, chemical interactions, and the human respiratory system into a single visual reference.

Chart structure: the x-axis is 0–100% relative humidity, the y-axis is the relative intensity of 7 different health risk parameters. Each parameter is drawn as a band; thicker bands indicate high risk at that RH range, while thinner bands indicate low risk.

The 7 parameters examined: 1. Bacterial growth (gram-positive, gram-negative) 2. Viruses (especially influenza, RSV, coronaviruses) 3. Fungal/mold growth (Aspergillus, Penicillium, Cladosporium) 4. Mite (Dermatophagoides) populations 5. Respiratory tract infections (epidemiological data) 6. Allergic rhinitis and asthma 7. Chemical reactions (formaldehyde emissions, metal corrosion)

The key finding of the chart: all 7 parameters reach minimum risk in the 40–60% RH range. This range has therefore been adopted as the "ASHRAE optimal indoor humidity zone" and is the fundamental target of modern HVAC design.

Research after 2010 (especially studies conducted during the COVID-19 period) has largely confirmed Sterling's findings. Studies published in NEJM and Lancet show that low (<30%) and high (>65%) RH environments increase the stability of viral aerosols.

Risk Bands

Sterling Chart risk bands (RH range vs effect):

Low RH (<30%): • Virus survival: HIGH • Respiratory infections: HIGH • Mucosal irritation: HIGH • Static electricity: HIGH • Asthma/allergies: HIGH • Bacterial growth: MODERATE • Mold growth: LOW • Mite population: LOW

Optimal range (40–60%): • All parameters: LOW risk • This range is the comfort zone of ASHRAE Standards 55 and 62.1

High RH (>70%): • Bacterial growth: HIGH • Mold growth: VERY HIGH (>70% RH) • Mite population: VERY HIGH (>65% RH) • Building material degradation: HIGH • Corrosion: HIGH • Virus survival: MODERATE-HIGH • Respiratory comfort: REDUCED

Critical thresholds: • Below 30%: viral aerosol density increases • 50%: minimum bacterial growth, optimal • 60%: ASHRAE comfort upper limit • 70%: mold growth threshold (sustained) • 80%: building material damage threshold • 85%: condensation + serious mold growth

Practical Example

Application of the Sterling Chart in a hospital HVAC design:

In the post-pandemic period, a hospital Infection Control Committee set new humidity targets. Current state analysis:

Zone — Current RH — Sterling Risk Assessment: • ER waiting room: 25% RH (after heating in winter months) → HIGH virus/respiratory risk • Intensive care: 35% RH → MODERATE-HIGH virus risk • General ward: 50% RH → OPTIMAL ✓ • Operating room: 55% RH → OPTIMAL ✓ • Laundry: 75% RH → HIGH mold + mite risk • Storage: 80% RH → VERY HIGH mold risk

Improvement plan: 1. ER and intensive care: additional humidification in winter. Steam humidification system (NKT humidifier) targeting 45 ± 5%. 2. Laundry: condensation-type dehumidifier targeting 60% (structural heat extraction + moisture reduction). 3. Storage: condensation type + increased ventilation, targeting 55%.

Expected impact: • 15–25% decrease in HAI (Hospital-Acquired Infection) rates (per CDC and other studies) • 10–18% decrease in staff sick days • Significant reduction in mold-driven building maintenance burden (wall/ceiling repair frequency drops noticeably)

Project investment class: medium-to-high CAPEX within the scope of hospital HVAC modernization Estimated payback: 14–18 months (compounded effect of HAI reduction + lower maintenance burden + recovered staff sick days)

In this project, the Sterling Chart is not a slide but the foundation of real engineering decisions and ROI calculations.

Engineering Note

Considerations in applying the Sterling Chart:

• 40–60% target is recommended, not absolute — sector/application-based preferences: – Healthcare (hospitals, ICUs): 45–55% (narrower band) – Comfort (offices, hotels, malls): 40–60% – Museums, archives: 50 ± 5% (preservation of artifacts) – Industrial processes: product requirement takes priority (chocolate <50%, textiles >55%, lithium battery <1%) • Seasonal variation — in winter, low-RH outdoor air combined with heating can drop indoor RH below 25%; in summer, high-RH outdoor air with insufficient cooling can rise above 70%. A season-based strategy is essential. • Local variations — even when the same setpoint is targeted, actual humidity distribution is not homogeneous in a building. Local variation can be ±10% near doors, exterior walls, and below HVAC diffusers. Multiple sensors + circulation are required. • Post-Sterling research — later studies (e.g. Noti et al., 2013, NIOSH/CDC) showed that the infectivity of airborne influenza virus falls markedly in the mid-humidity band. These findings are consistent with Sterling's 1985 assessment. • Energy optimization — narrow RH bands (e.g., 50 ± 2%) consume high energy; should be balanced with ASHRAE 90.1 energy-saving measures. A wider band (40–60%) is optimal for comfort + energy. • Continuous monitoring — the Sterling Chart is a design guide; actual operational performance must be verified via BMS monitoring and trend analysis.

ASHRAE Fundamentals and the Sterling chart can be used together as references for design decisions.