Air Quality in Pig Housing: Welfare and Management

Air Quality and Pig Welfare

Air quality is one of the most significant determinants of pig welfare in indoor housing systems. Poorly ventilated, overcrowded pig housing creates an environment that causes respiratory disease, chronic stress, and significant welfare compromise. Unlike temperature, which pigs and stockpersons can both directly sense, air quality problems — high ammonia, elevated carbon dioxide, dust, and pathogens — are often invisible until their effects manifest as disease and poor performance.

Key Air Quality Parameters

Ammonia (NH₃)

Ammonia is produced from decomposition of pig urine and faeces. It is the most significant air quality welfare concern in pig housing:

Welfare threshold: Maximum 20 ppm (UK Welfare of Farmed Animals Regulations)
Industry target: <10 ppm at pig level
Effects at 10–20 ppm: Ciliary damage in respiratory tract, reduced nasal mucociliary clearance, increased pathogen susceptibility
Effects at >25 ppm: Ocular irritation, reduced feed intake, increased respiratory disease incidence
Chronic exposure: Associated with higher pneumonia prevalence and increased antibiotic use

Carbon Dioxide (CO₂)

Maximum: 3,000 ppm (regulatory limit)
Target: <1,500 ppm for optimal welfare
High CO₂ indicates inadequate fresh air exchange — often a proxy for other air quality problems

Hydrogen Sulphide (H₂S)

Produced during slurry agitation; levels spike dramatically when slurry is disturbed
Maximum 5 ppm (regulatory); potentially fatal at >50 ppm
Evacuate building before slurry agitation; ventilate thoroughly

Particulate Matter

Dust (skin cells, feed particles, dried faeces) carries pathogens and irritates respiratory epithelium
Target: <3.4 mg/m³ total suspended particles
Straw-based systems typically have higher dust loads; management of bedding quality important

Ventilation Principles

Ventilation must balance:

Air quality: Sufficient fresh air exchange to dilute ammonia, CO₂, moisture, and pathogens
Temperature: Adequate warmth for young/recently weaned pigs without excessive energy use
Draughts: Avoiding chilling, particularly at pig level for young stock

Minimum ventilation rate guidelines:

Weaner pigs (7–30 kg): 0.7–3 m³/hour/pig (minimum–maximum)
Grower pigs (30–70 kg): 3–10 m³/hour/pig
Finisher pigs (70–110 kg): 5–20 m³/hour/pig
Sows: 10–50 m³/hour/sow

Practical Management Interventions

Slurry management: Remove slurry regularly (at least weekly); use pit ventilation to draw ammonia away from pigs before it rises
Bedding: Maintain dry, clean bedding — wet, soiled bedding is the primary ammonia source in straw-based systems
Stocking density: Overcrowding concentrates ammonia and reduces per-animal ventilation capacity
Roof and air inlet design: Ensure fresh air enters at high level and distributes without draughting pigs
Fan maintenance: Clean fans and inlets monthly; fan failure in summer causes rapid heat and ammonia build-up
Monitoring: Use ammonia meter at pig level monthly; CO₂ monitoring provides early warning of ventilation problems

Welfare Impact Summary

Poor air quality in pig housing represents a chronic welfare burden. Unlike acute stressors, poor air quality causes gradual, cumulative harm:

Chronic respiratory irritation and subclinical disease
Increased susceptibility to enzootic pneumonia, PRRS, and swine influenza
Immunosuppression reducing resistance to all diseases
Eye irritation from high ammonia
Reduced feed intake and growth rates