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Pollutants explained
The US EPA AQI is calculated from the highest sub-index across six criteria pollutants. The two that drive most Cayman AQI readings are PM2.5 — fine particulate matter, including the Saharan dust transported across the Atlantic each summer — and ground-level ozone on hot, sunny afternoons.
| Pollutant | What it is | Main sources | Primary health impact |
|---|---|---|---|
| PM2.5 Fine particulate matter | Inhalable particles 2.5 micrometres or smaller — small enough to reach the deepest parts of the lung and the bloodstream. | Vehicle exhaust, diesel generators, ship emissions, construction dust and Saharan dust transported across the Atlantic. | Long-term exposure is linked to cardiovascular disease, reduced lung function and premature mortality. |
| PM10 Coarse particulate matter | Inhalable particles 10 micrometres or smaller, deposited mainly in the upper airways. | Road dust, construction sites, sea spray, mould spores and again Saharan dust events common in Cayman summers. | Aggravates asthma and bronchitis; causes coughing and short-term respiratory irritation. |
| O₃ Ground-level ozone | A reactive gas formed when sunlight drives chemical reactions between nitrogen oxides and volatile organic compounds. | Not directly emitted — produced photochemically from vehicle and industrial precursors, peaking on hot, sunny afternoons. | Reduces lung function, triggers asthma attacks and inflames the airways even in healthy adults. |
| NO₂ Nitrogen dioxide | A reddish-brown gas produced by high-temperature combustion. | Vehicle engines (especially diesel), power generation and shipping. Highest near busy roads. | Inflames the lining of the lungs and lowers immunity to respiratory infections; a key driver of childhood asthma onset. |
| SO₂ Sulphur dioxide | A colourless gas with a sharp odour, produced when sulphur-bearing fuels are burned. | Diesel power generation, heavy fuel oil used by ships at anchor and industrial combustion. | Constricts the airways and triggers wheezing, especially in people with asthma. Contributes to acid rain. |
| CO Carbon monoxide | A colourless, odourless gas produced by incomplete combustion of carbon-based fuels. | Petrol engines, gas appliances, generators and cooking fires. Most outdoor exposure comes from traffic. | Reduces oxygen delivery to the heart and brain. Acutely dangerous indoors; outdoors a concern only at very high concentrations. |
Index by EPA category
The official US Environmental Protection Agency Air Quality Index runs from 0 to 500 and is divided into six colour-coded categories. Cayman’s background AQI sits comfortably in the Good band; the categories above exist primarily for context during Saharan dust events and very high ozone days.
| AQI | Category | Health implications |
|---|---|---|
| 0–50 | Good | Air quality is satisfactory and poses little or no risk. A normal day to be outside. |
| 51–100 | Moderate | Acceptable for most. Unusually sensitive people should consider reducing prolonged or heavy outdoor exertion. |
| 101–150 | Unhealthy for Sensitive Groups | Members of sensitive groups — children, older adults, people with heart or lung disease — may experience health effects. |
| 151–200 | Unhealthy | Some members of the general public may experience health effects; sensitive groups may experience more serious effects. |
| 201–300 | Very Unhealthy | Health alert: the risk of health effects is increased for everyone. Reduce outdoor activity. |
| 301+ | Hazardous | Health warning of emergency conditions: everyone is more likely to be affected. Remain indoors. |
Frequently asked questions
What does the AQI mean for the Cayman Islands?
The Cayman Islands sit in the open Atlantic and benefit from steady trade winds, so background AQI is usually in the Good (0–50) band. Episodic spikes are driven mostly by Saharan dust events between June and August, by ship and cruise emissions in George Town harbour, and by traffic on the main arterial roads through Seven Mile Beach and George Town.
How often is the AQI on this page updated?
Air-quality observations on this page refresh every 15 minutes from the same upstream feed used by the rest of the Weather Desk. The US EPA index reported is the same 1–6 scale published by the National Weather Service and is computed from concurrent PM2.5, PM10, ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide measurements.
Who counts as a "sensitive group"?
The US EPA defines sensitive groups as children, adults over 65, pregnant people, anyone with asthma, chronic obstructive pulmonary disease, other lung conditions, heart disease or diabetes, and outdoor workers with sustained exposure. These groups should begin to limit prolonged outdoor exertion once the AQI reaches 101 (Unhealthy for Sensitive Groups).
Does outdoor air quality affect indoor air?
Yes — indoor PM2.5 typically tracks outdoor PM2.5 within a few hours, especially in homes with open windows or natural ventilation, which is most Cayman housing stock. During Saharan dust events or very high AQI readings, close windows, run air-conditioning on the recirculate setting, and consider a portable HEPA filter for sleeping areas.
When should I limit outdoor activity?
Healthy adults can typically continue normal outdoor activity through the Good and Moderate bands (AQI 0–100). At Unhealthy for Sensitive Groups (101–150), sensitive groups should reduce prolonged or heavy exertion. At Unhealthy (151–200) everyone should reduce prolonged exertion; from Very Unhealthy (201+) outdoor exercise should be avoided.
Why we keep this page
The Cayman Islands have some of the cleanest background air anywhere in the developed world — open Atlantic on every side and a steady trade wind. But background is not the same as constant. Cruise traffic, harbour generators, peak-hour congestion through George Town and the seasonal Saharan dust plumes that arrive every June all pull the AQI off baseline for hours or days at a time. The Weather Desk maintains this page so that residents — particularly those in sensitive groups — have a single, on-record reference for what the air is doing right now.
Readings are computed from the same upstream feed as the rest of the Weather Desk and refresh every 15 minutes. If something on this page disagrees with an official notice from the Department of Environmental Health, follow the official notice and write to us.