The PICE Index on environmental quality for the 10 most-visited cities in the world

Matthew Keogh

Sep 29, 2020

The environment is humanity’s collective home.

So, it’s no surprise that when its quality is poor, activity driven by its inhabitants could give us an indication as to why.

A city’s environmental quality provides an indication of different disturbances in the form of pollutants and wastes, that determines the extent of environmental deterioration in the area.

To provide a better understanding of how pollution indicators affect cities’ environmental quality, we’ve created the ‘Pollution Indicators on City Environments (PICE) Index’ for the 10 most-visited cities in the world.

We’ve also asked some big questions, such as:

  • what contributes to poor environmental quality;
  • what are the future implications on travel; and
  • what’s currently being done to improve the environment – all while trying to gain a better understanding of how all these cities stack up against each other environmentally.

The following 10 cities have been chosen because they represent the 10 most-visited cities in the world, according to Euromonitor.1

What is the PICE Index?

The PICE Index measures a number of pollution indicators that contribute to a cities’ environmental quality – each data point (component) represents a different pollution indicator. The overall score for each city is calculated by their performance across each component, which is given a weighted score out of 60 to represent a condensed look at the original data. The components are:

  • number of cattle by country of city (2018)2
  • number of poultry birds by country of city (2018)3
  • air quality PM2.5 (annual average 2019)4
  • population density per km2 (2019)5
  • annual visitors (2019)6
  • most traffic congestion world city ranking (2019)7

Read our detailed methodology for more information.

What’s better: a higher or lower index score?

We expect that a lower overall score should correlate with a better environmental quality. As indicated, London performs the best while Hong Kong performs the worst.

Key points from the PICE Index

Neighbouring cities’ Hong Kong and Macao record the worst overall score. While Macao has almost seven million fewer people living in its area than Hong Kong, they still operate under similar processes and government regulations. This may indicate why they almost match Hong Kong’s score. For instance, Macao and Hong Kong are a part of the same air quality monitoring network overseen by the Environmental Protection Department.

The high scores (comparative to the other cities) may be attributed to China’s vast population and production of goods. Geographically, Hong Kong and Macao are separated by the Pearl River Delta – one of the busiest port gateways in the world.

London records the best score. Once an economic powerhouse, London (and the United Kingdom as a whole) don’t produce nearly as many materials and live exports as most of the other cities and countries listed above. The reduction in production of goods seems to have decreased their scores (including traffic congestion).

Dubai has the highest PM2.5 level (of the included countries) which increases their overall score. They also record the highest level of traffic congestion. According to the International Association for Medical Assistance to Travellers (IAMAT), Dubai consistently records high levels of air pollution from industrial emissions and vehicle emissions.8

Each city has its own specific problems with environmental quality. Whether it’s high emissions from industrialisation (Hong Kong and Macao), cross-state pollution from high production of animal agriculture (*New York) or relatively high PM2.5 levels (Dubai, Kuala Lumpur and Bangkok), each city has their own specific problem that affects their overall score. This indicates that environmental quality is a multivariate problem with no ‘one size fits all’ solution.

*While New York City isn’t a major animal agriculture production hub, they are affected by cross-state air pollution from larger production states like Maryland, Illinois and Virginia.9

How is air quality measured?

The United States Environmental Protection Agency Air Quality Index (US AQI) is one of the most recognised indexes for communicating air quality. The US AQI converts pollutant concentrations (PM2.5) into a colour-coded scale of 0-500. Higher values indicate a higher level of air pollution.

How does our score differ from the US AQI? Our score factors in PM2.5, plus several other factors. So, it adds another level of data via pollution indicators like animal agriculture and traffic congestion to illuminate the specific problems pertaining to each city.


Index Value: 0 – 50

PM2.5(ug,m3): <15

Air quality is considered satisfactory, and air pollution poses little or no risk.


Index Value: 51 – 100

PM2.5(ug,m3): 16 – 40

Air quality is considered satisfactory, and air pollution poses little or no risk.

Unhealthy for Sensitive Groups

Index Value: 101 – 150

PM2.5(ug,m3): 41 – 65

Members of sensitive groups may experience health effects. The general public is not likely to be affected.


Index Value: 151 – 200

PM2.5(ug,m3): 66 – 150

Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.

Very Unhealthy

Index Value: 201 – 300

PM2.5(ug,m3): 151 – 250

Health alert: everyone may experience more serious health effects


Index Value: 301 – 500

PM2.5(ug,m3): >251

Health alert: Health warnings of emergency conditions. The entire population is more likely to be affected.

What is PM2.5?

PM stands for ‘particle matter’, and 2.5 represents ambient airborne particles measuring up to 2.5 microns in size (that’s 24 times smaller than the diameter of a human hair). Its microscopic size allows the particles to move throughout the body via the bloodstream, causing health effects, including asthma, lung cancer and heart disease.

What’s causing air pollution?

According to WHO,10 the most significant contributing factors to poor air quality are:

    • transport
    • waste management
    • industry & energy supply
    • dust
    • agriculture practices
    • household energy.

So how do these elements manifest into the air? Well, combustible fossil fuels like coal, petroleum and natural gas caused by transport and other industries release carbon dioxide and other greenhouse gases – which react with sunlight and create smog and soot in the air.

One industry (in particular), animal agriculture, can be linked to every one of the WHO’s most significant contributing factors listed above. According to the WHO, animal-sourced foods tend to be associated with higher greenhouse gas emissions, such as methane and nitrogen dioxide.11 Animal food industries require large amounts of energy and resources to raise, maintain and distribute animal products, thereby causing a strain on the environment.

On an individual level, people contribute to emissions through transport, heating, cooling and waste. Furthermore, natural disasters can affect air quality. Bushfires and volcanic ash (especially) have a detrimental effect. All in all, what’s clear from the WHO’s data is that poor air quality is mainly due to the industrial burning of fossil fuels – coal, oil and gas – and exacerbated by deforestation.

How popular cities are affected and the actions they are taking

Tourism is incredibly important for many popular cities who rely on annual visitors for economic growth. This could form a specific correlation with pollution indicators, particularly as 2020 saw the WHO nominate poor air quality as the single biggest environmental health crisis. Consequently, as air quality becomes more of a travel consideration, cities who take improving air quality seriously may be better placed to avoid loss of tourist numbers in the coming years.

For instance, Dubai – a popular tourist destination – is experiencing high levels of air pollution. According to the United Arab Emirates Ministry of Climate Change & Environment, this is due to high usage of the United Arab Emirates (UAE) natural resources and increased demand from population growth and tourism.12 However, it’s not only Dubai that suffers from air pollution. The previous factors cause air pollution for most cities rich in natural resources and experiencing growth. For example, Hong Kong and Macao are recording high PM2.5 levels through their economic growth. As previously mentioned, both cities are shipping hubs and rely on international exports for a healthy economy.

Although, one economic powerhouse is leading the way in terms of PM2.5; New York City is consistently acting to lower their levels year on year. One of the biggest cities in the world by population and visitors, New York City recorded the best average PM2.5 level out of the cities on our list. According to the New York City Community Air Survey, the improvement stems from the reduction of sulphur dioxide emissions – due to City and State heating oil regulations.13 Furthermore, the Clean Air Act of 1970 requires the Environmental Protection Agency (EPA) to regulate the emissions of large industrial facilities in every U.S. city. According to the EPA, from 1970 to 2017, aggregate national emissions of the six common pollutants alone dropped an average of 73 per cent.14

Many other of the world’s most visited cities have their own regulations to maintain a ‘reasonable’ air quality standard. Most European cities have Air Quality Action Plans aimed at improving air quality, with each city action plan tying in with national, regional and European Union (EU) measures.

Changes to the city landscape and how we live

According to The Organisation for Economic Co-operation and Development (OECD),15 the three different market impacts of air pollution are: reduced labour productivity, increased health expenditures and crop yield losses.

All three of these market impacts could affect cities to varying degrees, and as a result, tourism may be detrimentally affected. Countries, cities and businesses may need to embrace more eco-friendly policies and renewable energy technologies – otherwise alternative destinations with better air quality may become a more popular travel choice.

However, it seems some cities are getting the message. For example, London is phasing out diesel-fuelled vehicles. In their place, more energy-efficient vehicle regulations, and hybrid-fuelled buses are being introduced.16 This initiative aims to reduce emissions and ease traffic congestion. This may be why many other cities around the world have introduced Bus Rapid Transport (BRT). BRT uses segregated bus-only lanes to transport daily commuters more efficiently. The cities that integrate systems like the BRT, plus subways, walking and biking could be best positioned to ease congestion, according to the 2019 Deloitte City Mobility Index.17

The energy and livestock industries are also getting a shake-up. The Canadian Government recently announced a $150 million investment in plant-based protein.18 Microsoft founder, Bill Gates is investing in plant-based meat while fellow Billionaire, Elon Musk is developing the mass production of electric-powered cars and solar panels.

Addressing the air pollution problem on a world scale

The big players in geo-politics are in the cut and thrust of tackling air pollution through a variety of schemes. For example, the Paris Agreement is a policy signed by more than 190 countries within the United Nations (UN) framework aimed at reducing greenhouse gas emissions.

The WHO makes the following suggestions for better air quality:19

    • invest in energy-efficient power generation;
    • improve domestic, industry and municipal waste management;
    • implement affordable public transport and pedestrian/cycle-friendly networks;
    • promote better waste management and agricultural practices;
    • provide universal access to cleaner and affordable fuels; and
    • promote greener and more compact cities with energy-efficient buildings.

The WHO is just one world organisation making an impact. In 2001, the UN started The Task Force on Techo-Economic Issues (TFTEI). The task force is responsible for collecting and assessing data on emissions to help reduce air pollutants and drive technological innovation around the world.

Scaling it back to a national level, our 10 most-visited countries each have their own complex problem with air pollution. Here we outline their problems and how they’re addressing them.

Hong Kong

In Hong Kong, air pollution is a major concern. Two key challenges are:

  1. Street-level pollution. Diesel vehicles, particularly trucks, buses and light buses, are the main source of street-level pollution.
  2. Smog. A mixture of smoke and fog, smog is caused by a combination of pollutants mainly from motor vehicles, industry and power plants in Hong Kong and the Pearl River Delta.

As of 2018, a comprehensive program lowered the roadside levels of:

    • nitrogen dioxide by 17 per cent;
    • respirable suspended particulates by 57 per cent;
    • fine suspended particulates by 54 per cent; and
    • sulphur dioxide by 74 per cent.

Source: Air quality in Hong Kong. Hong Kong Government. GovHK. 2020.


Bangkok and surrounding regions in Thailand have air pollution issues. In recent times, Bangkok has experienced toxic smog from traffic, construction works, crop burning and factory pollution.

The United Nations (UN) Environment and the Climate and Clean Air Coalition are currently working with the Thailand Pollution Control to improve air quality.

Source: Air pollution is choking Bangkok, but a solution is in reach. Environment Programme. The United Nations. 2020.


One of the main contributors to the air pollution in Macao are the manufacturing factories from several industrialised regions across the border and within the Pearl River Delta – one of the world’s largest shipping hubs.

Macao, Guangdong and Hong Kong are a part of an air quality monitoring network overseen by the Environmental Protection Department. The Regional Air Monitoring Network aims to tackle vehicle and power station emissions and improve the Pearl River Delta region.

Source: Pearl River Delta Air Quality. Hong Kong Government. GovHK. 2020.


Air pollution in Singapore primarily stems from industry and motor vehicles. Periodically, Singapore experiences transboundary smoke haze from land and forest fires near the region.

To control emissions, the National Environment Agency (NEA) established the Advisory Committee on Ambient Air Quality in July 2010 to recommend air quality targets for Singapore.

Source: Air quality. Air Quality in Singapore. National Environment Agency. A Singapore Government Agency website. 2020.


Since his election in 2016, London’s Mayor Sadiq Khan has led an active campaign to combat poor air quality in London. His plan includes an initiative called the Ultra Low Emissions Zone (ULEZ), a system designed to reduce emissions and ease congestion.

A large component of London’s air pollution is caused by road transport. To help lower emissions, ‘Transport for London’ recently introduced 2,600 diesel-electric hybrid buses.

Source: Environment. Pollution and air quality. Greater London Authority. 2020.


In a recent court case, a woman and daughter (with respiratory problems) successfully sued the state for air pollution in a landmark case. According to the ruling, the French state failed to do enough to limit air pollution around city.

For years, Paris has experienced smog due to transport, heating and industry. They’ve since implemented cheaper public transport and other measures to mitigate the effects of air pollution on its citizens.

Source: France court holds Government liable in air pollution case. Legal News and Research. Jurist. 2020. Regulation of air pollution: France. Library of Congress. 2020.


Cities within the United Arab Emirates experience high pollution, partly due to the exportation of natural resources, population growth and high energy demand. As a result, Dubai has an air quality station that warns locals when dust and smog levels become too high.

Historically, Dubai is not the biggest investor in environmental sustainability, however the regional government aims to have 90 per cent clean air days by 2021. They aim to achieve this through their Air Quality Strategy.

Source: Dubai Air Quality Strategy. Dubai Environment. Government of Dubai. 2020.


Istanbul is Turkey’s most polluted city and lags in clean air standards and monitoring pollutants. One of the main causes is the use of low-quality coal for the transportation and heating.

In 2019, an initiative led by citizens, health professionals and organisations such as the Right to Clean Air Platform caused the Turkish Parliament to withdraw a proposal for emissions exemptions for privatised coal-fired power plants.

Source: Turkish Health Professionals call for Better Air Pollution Monitoring and Legislation. Health and Environment Alliance (HEAL). 2020.

Kuala Lumpur

The slash and burn agricultural technique (cutting and burning trees in a forest) employed by Malaysia and neighbouring Indonesia is one of the causes of Kuala Lumpur’s air pollution problem.

In recent years, air pollution from forest fires have caused disruptions to the Malaysian capital’s economy and the health of its people and wildlife. In 2014, Singapore passed the Transboundary Haze Pollution Act, allowing its government to prosecute companies operating in Indonesia that cause air pollution in Singapore.

Source: Can Malaysia do anything about its poor air quality? The main causes of Malaysia’s poor air quality. Geographical association. 2020.

New York City

New York City experiences cross-state air pollution from states like Maryland, Illinois and Virginia. The polluted air from agriculture, coal and oil industries, travels downwind to New York City.

In 2011, the Environmental Protection Agency (EPA) introduced the Cross-State Air Pollution Rule (CSAPR) to mitigate air pollution that crosses state borders and alters air quality in downwind states.

Source: Cross-state Air Pollution Rule (CSAPR). United States Environmental Protection Agency. 2020.

How new technology could save the day

New technologies may help solve some of our current air pollution problems. Several new technologies (currently in development) could improve air quality. They include:

  • renewable energy, e.g. solar energy, hydroelectric power, wind energy
  • electric vehicles
  • hydrogen fuel additives to improve the fuel combustion in existing vehicles
  • autonomous vehicles, which could improve fuel efficiency
  • ‘Smog Free Towers’ – air-purifying towers that inhale air pollution and expel clean air
  • ‘liquid air’ to power high pollutant appliances, such as refrigerated vans and trucks.

The world is full of change and innovation, from the invention of the wagon wheel to the first man on the moon. That’s why it’s inevitable that how we live now will differ for future generations.

Something else that’s always changing is the most visited cities in the world list. So, once we can travel again and before you lock-in your next travel destination, compare travel insurance. It’ll help keep you safe when travelling abroad and give you that added peace of mind.


The banded score used in this article is a method that generates a score from multiple key data points to determine an indication of a cities’ air cleanliness. Each city’s data points (components) are assigned to a band that aligns with their particular data. This objective band data represents a condensed look at the original data.

In order to calculate these bands, objective data ranges were created that represent the current component sizes for each city while allowing room for growth in the future. Band ranges are increased to larger amounts at certain points as the bands get higher to allow for scaling of the data. The number of bands can extend beyond ten where needed, based on the continuous ascending value pattern. In order to represent components with a lower effect on air quality based on the research in this article, some bands have a wider range which reduces the effect on the overall score.

The overall score for each city is the band sum of all components. As Macao only has data for five of the six components, its overall score is increased by 16.6% to compensate.

We expect that a lower score will correlate with a better environmental quality and amenity.

The below table shows the original data in numeric form which serves as a basis for the banded scores. Each component represents a data point that has an effect on environmental quality and pollution, as described in the research of the article.

Components used to develop a score are number of cattle by country of city (2018), number of poultry birds by country of city (2018), air quality PM2.5 (annual average 2019), population density per km2 (2019), annual visitors (2019) and most traffic congestion world city ranking (2019).


Top 10 most travelled cities’ numeric data (before banding)

The table below shows the original data for each component, before being banded into the PICE Index.

Band allocation of each component

The table below shows the allocation of figures for each band and component. The data represented indicates the value range of each band per component.



  1. Euromonitor. Top 100 city destinations: 2019 edition. 2019.
  2. Our World in Data. Meat and Dairy Production. Beef and buffalo (cattle) meat production. Poultry production. 2018. Accessed May 2020.
  3. IBID.
  4. World’s Air Pollution. Real-time Air Quality Index. 2020. Accessed May 2020.
  5. World population review. World City Populations 2020. 2020. Accessed May 2020.
  6. Euromonitor International. Top 100 City Destinations: 2019 Edition. 2019. Accessed May 2020.
  7. TomTom. Traffic Index 2019. 2019. Accessed May 2020.
  8. International Association for Medical Assistance to Travellers (IAMAT). United Arab Emirates. 2020.
  9. Environment Defense Fund. EPA Denies New York’s Request for Help with Cross-State Air Pollution. 2019. Accessed August 2020.
  10. World Health Organisation. Infographic: what are the sources of air pollution? 2020.
  11. World Health Organisation. Air pollution. Interventions: agriculture. 2020.
  12. United Arab Emirates Ministry of Climate Change & Environment. UAE National Air Emissions Inventory Project. 2019.
  13. The official website of New York City. New York City’s Air is Cleaner Than It Has Ever Been Since Monitoring Began. 2018.
  14. United States Environmental Protection Agency. Progress Cleaning the Air and Improving People’s Health. Detailed Summary: Clean Air Act Results. 2020.
  15. The Organisation for Economic Co-operation and Development. Policy highlights: Economic consequences of outdoor air pollution. 2016.
  16. Cleaner buses. Mayor of London. London Assembly. 2020.
  17. Deloitte. The 2019 Deloitte City Mobility Index. 2019.
  18. Plant based news. Canadian Government to Invest $150 Million In Plant-Based Sector. 2018.
  19. World Health Organisation. Air pollution infographics. 2020.

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