Introduction

Did you know that World Health Organisation (WHO) data reveals that air pollution is a pervasive issue, affecting almost every person on the planet (99% of the global population), with the situation being particularly severe in less affluent countries? This widespread exposure to polluted air, which breaches the WHO's safety standards, poses significant health risks to populations worldwide. 

Pertussis is recognised as one of the established health hazards linked to air pollution. A study conducted in 31 provinces of China identified a connection between an elevated monthly exposure to carbon dioxide and ozone and higher instances of pertussis. Additionally, particulate matter(PM)_2.5 and sulphur dioxide were also found to contribute significantly to the heightened risk of pertussis. The findings from this study, amongst others,  find a connection between pertussis incidence and air pollution.¹ It has been proposed that public health practitioners should consider environmental risks when devising strategies for pertussis prevention and control.¹ 

Understanding pertussis

Pertussis or whooping cough, which we have known for more than a century, is a highly contagious and life-threatening respiratory system disease caused by bacterial infection of the lungs and airways. It has an impact, especially on newborns, pregnant people, and the elderly.² 

It initially resembles a common cold but progresses into violent, uncontrollable coughing fits, accompanied by difficulty breathing. These severe coughing spells can last for an extended period, potentially leading to vomiting, as well as rupturing of blood vessels in the eyes and on the skin's surface. The characteristic "whoop" sound often occurs when the person desperately gasps for air after a coughing episode.³ 

The symptoms usually appear within 5-10 days after exposure to the Bordetella pertussis bacteria. In the early stages, whooping cough can seem like a regular cold, with:³

• Congested/runny nose
• Mild fever under 100.4°F
• Occasional mild cough (though infants may not cough at this point)
• Infants/young children may experience pauses in breathing and bluish skin discolouration

Because the initial symptoms mimic a common cold, doctors often overlook or misdiagnose whooping cough until more severe symptoms develop later on. The disease can be mistaken for a routine illness in these early stages, before the telltale severe coughing fits characteristic of pertussis emerge.³

The Bordetella pertussis bacteria are highly transmissible and spread very easily from person to person through the air. When someone with whooping cough coughs or sneezes, they expel tiny airborne droplets carrying the bacteria. Others can then inhale these bacteria-laden droplets and become infected. Transmission also occurs by spending extended periods close to or sharing breathing spaces with an infected individual.^1,3 

After the introduction of the whole-cell pertussis vaccine in the 1930s, there have been great advances in controlling the disease spread, however, pertussis continues to be a problem for vulnerable individuals.¹

Types of air pollution

According to the WHO, some of the most hazardous air pollutants for public health include particulate matter (PM), carbon monoxide, ozone, nitrogen dioxide, and sulphur dioxide. These pollutants pose risks to health in both the short and long term, with some having no safe exposure level below which harmful effects do not occur.

• Particulate Matter (PM) consists of tiny inhalable particles like sulphates, nitrates, carbon, dust, and varies in size. The smallest PM_2.5 and PM₁₀ are especially concerning, linked to cardiovascular disease, stroke, respiratory issues, adverse pregnancy outcomes, and lung cancer from long-term exposure
• Nitrogen Dioxide (NO₂), a reddish-brown gas from combustion in transportation, homes, and industry, can irritate airways and exacerbate respiratory conditions like asthma
• Ground-level Ozone (O₃), a key smog component, forms from vehicle and industrial emissions reacting with sunlight. Elevated ozone impairs breathing, triggers asthma attacks, reduces lung function, and contributes to lung diseases
• Carbon Monoxide (CO), an odourless gas from incomplete fuel combustion, reduces oxygen delivery in the body, causing breathing issues, fatigue, dizziness, flu-like symptoms, and potentially fatal effects at high levels
• Sulphur Dioxide (SO₂), released from burning fossil fuels, is associated with asthma exacerbations and respiratory-related emergency room visits

Impact of air pollution on pertussis

Outbreaks of pertussis have occurred in multiple countries around the world, including China, the Philippines, the Czech Republic, the Netherlands, the United States, and the United Kingdom. Of particular note, China experienced a substantial spike in whooping cough cases during January and February 2024, with over a 20-fold increase compared to the same period in the previous year. Researchers hypothesise that one potential factor driving the rising incidence of pertussis over the past two decades in China may be the impact of outdoor air pollution.

An analysis revealed that levels of ozone (O₃), fine particulate matter (PM_2.5), and sulphur dioxide (SO₂) could modify the impact of NO₂ on pertussis incidence rates. In areas with high concentrations of PM_2.5, every one-unit increase in NO₂ corresponded to a 2.52% rise in pertussis incidence risk. However, in low PM_2.5 environments, the increase in risk was slightly lower at 2.16% for each one-unit NO₂ increase. When looking at SO₂ levels, a one-unit rise in NO₂ resulted in a 1.71% higher pertussis risk in low SO₂ environments versus a 1.34% increase in high SO₂ areas.⁴

For ozone, high O₃ concentrations amplified the effect, with a one-unit NO₂ increase leading to a 2.24% higher pertussis incidence risk. Notably, no significant change was detected in low O₃ environments. In summary, co-exposures to elevated PM_2.5, SO₂, and O₃ appeared to exacerbate the impact of increased NO₂ levels on raising the risk of pertussis infections across the studied populations. It suggests interactions between these pollutants in influencing whooping cough rates.⁴

Scientists are working to better understand the link between NO₂ exposure and respiratory illnesses. NO₂ is known to be an irritant to the airways and may interact with the immune system, potentially leading to respiratory infections and lung inflammation. Prolonged exposure to elevated NO₂ levels can exacerbate airway irritation and trigger inflammatory responses in lung tissue. Additionally, other pollutants like PM_2.5, SO₂, and O₃ have been implicated in compromising the airway barrier function and are associated with increased overall and respiratory mortality rates.⁴

Numerous animal studies have highlighted the significant detrimental impact of O₃ exposure on respiratory health with demonstrated outcomes such as lung inflammation, emphysema, and oxidative stress. A study to comprehend the relationship between short-term air pollution exposure and lung function indicated that PM_2.5 and NO₂ were contributing factors to decreased lung function capacity.⁴

The USC Children's Health Study conducted assessments of lung development in children between the ages of 11 and 15. The results showed significant improvements in lung function for the group of children examined from 2007 to 2011 compared to earlier cohorts from 1994-1998 and 1997-2001 living in the same communities. These gains in lung capacity corresponded with enhancements in air quality within those communities and across the Los Angeles basin, which were attributed to the implementation of air pollution control policies. In this study, the combined exposure to two harmful pollutants - NO₂ and PM_2.5 - decreased by approximately 40% for the 2007-2011 cohort compared to the 1994-1998 group. Notably, as air quality improved, children's lung growth accelerated. Lung development between ages 11-15 showed over a 10% greater increase for children breathing lower NO₂ levels from 2007-2011 versus those exposed to higher levels from 1994-1998.

Summary

In summary, pertussis is a highly contagious bacterial respiratory infection that starts mild but develops into severe coughing fits, spreading easily through coughing/sneezing droplets. Studies have linked increased pertussis incidence to higher levels of air pollutants like NO₂, PM_2.5, O₃, and SO₂ in various countries, including China. NO_2, along with PM_2.5, SO₂, and O₃, have been identified as harmful air pollutants that can irritate airways, promote inflammation, impair lung function, and increase respiratory disease risks through various mechanisms, supported by human epidemiological studies and animal experiments. Children's lung function capacity improved substantially, influenced by better air quality resulting from pollution control measures in Los Angeles.