Indoor Air Quality

Indoor air is often more polluted than outdoor air because people spend 80-90% of their time indoors and pollutants build up where ventilation is poor.

Classification of sources

• Building materials and furnishings: formaldehyde, asbestos, radon, VOCs.
• Combustion: cooking and heating appliances.
• Occupants and activities: CO$_2$, bio-effluents, smoking, cleaning, hobbies.
• Outdoor air entering the building: vehicle exhaust, pollen, dust.
• HVAC systems: contaminated ducts, filters and humidifiers.

Sick building syndrome (SBS) is a set of non-specific symptoms experienced by occupants of a building that appear while they are in the building and disappear soon after they leave, and for which no specific cause or disease can be identified. It is mostly reported in air-conditioned, tightly sealed offices.

Symptoms

• Headache, fatigue, dizziness, difficulty concentrating
• Irritation of eyes, nose and throat, dry cough, skin dryness and itching
• Nausea and sensitivity to odours

Causes

• Inadequate ventilation and poor distribution of fresh air
• Chemical contaminants from furniture, carpets, paints, copiers and cleaning agents (VOCs, formaldehyde)
• Biological contaminants: mould, bacteria in ducts and drip trays
• Poor temperature and humidity control, poor lighting and noise, job stress

Building-related illness (BRI)

Here the symptoms can be diagnosed and traced to a specific cause in the building, and they may continue after leaving. Examples: Legionnaires' disease, hypersensitivity pneumonitis, humidifier fever, asthma from mould.

Remedies

• Increase fresh air supply and maintain the HVAC system (clean filters and ducts)
• Remove or reduce the source (low-VOC materials, no smoking)
• Control humidity at 30-60%, and clean regularly

Three basic strategies are used, in order of effectiveness: source control, ventilation, and air cleaning.

1. Source control (best and most economical)

• Eliminate the source, e.g. ban smoking, use only unleaded paints, avoid asbestos.
• Substitute with low-emission products: low-VOC paints, formaldehyde-free boards, electric or LPG stoves.
• Modify the source: seal radon entry cracks, enclose asbestos, allow new furniture to off-gas outdoors, vent combustion appliances outside through flues and hoods.
• Keep humidity below 60% to prevent mould, and fix leaks.

2. Ventilation

• Natural: windows, vents, and stack effect.
• Mechanical: exhaust fans in kitchens and toilets; supply and exhaust fans with heat recovery.
• Supply enough outdoor air per person according to ventilation standards (for example 8-10 L/s per person is typical).
• Place the intake away from outdoor pollution (traffic, exhaust).

3. Air cleaning

• Particle filters: fibrous filters, HEPA filters (99.97% of 0.3 $\mu$m), electrostatic precipitators.
• Gas removal: activated carbon adsorption for VOCs and odours.
• Cleaners treat only part of the air and do not remove all gases, so they supplement the other two methods.

Other measures

• Regular maintenance and cleaning of HVAC systems, test houses for radon, carbon monoxide detectors, and good housekeeping (wet cleaning, no dust traps).

(a) Ventilation standards

A ventilation standard (e.g. ASHRAE 62.1) specifies the minimum outdoor air that must be supplied to a space so that the concentration of contaminants stays at acceptable levels for occupants. It protects health and comfort and fixes the design of HVAC systems.

Ways of specifying:

1. Ventilation rate procedure: a prescribed airflow per person (L/s per person) plus a component per unit floor area (L/s per $\text{m}^2$) for building-related sources, depending on the use of the space.
2. Air change rate: the number of room volumes of fresh air supplied per hour (ACH), common for hospitals, kitchens and laboratories.
3. (Also) Indoor air quality procedure: by limiting the concentration of selected contaminants (e.g. CO$_2$ difference from outdoors of about 700 ppm, or total VOCs).

(b) Numerical

Mass balance at steady state: pollutant generation = removal by ventilation.

$$G = Q\,(C_i - C_o) \Rightarrow Q = \frac{G}{C_i - C_o}$$

Generation: $G = 10 \times 0.0052 = 0.052$ L/s $= 5.2\times10^{-5}\ \text{m}^3$/s.

Concentration difference: $C_i - C_o = (1000 - 400)\times10^{-6} = 6\times10^{-4}$.

$$Q = \frac{5.2\times10^{-5}}{6\times10^{-4}} = 0.0867\ \text{m}^3/\text{s} = 86.7\ \text{L/s} = 312\ \text{m}^3/\text{h}$$

Per person: $86.7/10 = 8.67$ L/s per person.

Room volume $= 6\times5\times3 = 90\ \text{m}^3$:

$$\text{ACH} = \frac{312}{90} = 3.47\ \text{h}^{-1}$$

Answer: Fresh air required = 312 m$^3$/h (86.7 L/s, i.e. 8.7 L/s per person); ACH = 3.5 per hour.

In many rural homes of developing countries, including the hill and Terai areas of Nepal, wood, dung and crop residue are burnt for cooking and heating on open fires or traditional mud stoves with no chimney. The smoke released inside poorly ventilated, often windowless kitchens gives very high exposure; women and young children are the most affected because they spend the most time near the fire.

Pollutants in biomass smoke

• Particulate matter (PM2.5 and PM10); levels of several hundred to thousands of $\mu\text{g/m}^3$ during cooking are common, far above guideline values.
• Carbon monoxide, nitrogen oxides, formaldehyde, benzene and other VOCs, and polycyclic aromatic hydrocarbons (PAH) such as benzo[a]pyrene (a carcinogen).

Reasons for high concentrations: incomplete combustion at low temperature, no chimney or hood, small rooms, low openings and poor ventilation.

Health effects

Smoke also darkens walls and roofs, and burning biomass adds to forest loss and the time spent collecting fuel.

Control measures

1. Improved cookstoves (ICS) with chimneys that vent smoke outside, giving better combustion.
2. Clean fuels: LPG, biogas, electric or induction cookers, solar cookers.
3. Ventilation: smoke hoods, windows, ventilators near the roof, and a separate kitchen.
4. Behaviour: use dry wood, keep children away from the fire, and cook outdoors where possible.
5. Awareness and policy: health education, subsidies for biogas and improved stoves.